[MatejKozlovsky]

Depends on #1523 (fx-owner, 13a49c9).

Root cause

With several frames in flight, VRAM fills up with resources we already let go of. A
gpu.mem=detail trace of 4K d_blurglow at 3 in flight, just before the out-of-memory error:

• The allocator held 45 layer-upload textures (6.7 GB). Only 6 still had a live GpuImage
  handle, all in the upload cache. The other 39 (~5.8 GB) were dropped on our side, but wgpu
  hadn't destroyed them yet: it does that only once the submissions that used them complete.
• About 3 GB more was in wgpu's internal staging buffers for queue writes, and 2.6 GB was pool
  textures owned by frames.
• Uploading into pooled textures instead only moved the problem: 14 GB of pool textures sat in
  "pending", and it hit more OOM errors.

So nothing leaks for good: frames in flight record work faster than the GPU retires it.

Fix

Enc::mem_gate runs after each effect call and each layer. Every 8th call it reads the wgpu
allocator report, and if more than the cap is allocated, the frame submits its encoder and waits
for the queue. wgpu then frees what was dropped, and pending pool textures become free.

• Cap: wgpu has no safe query for adapter memory (the workspace forbids unsafe). The default is
  8 GiB on discrete GPUs and 2 GiB on integrated/other adapters. EFFECTCRAFT_GPU_MEMCAP_MB
  overrides it; 0 turns it off.
• Backends without an allocator report (Metal, GL, WebGPU) are never gated.
• The first wait logs one info line.

Results

AMD Ryzen 7 5800X / NVIDIA GeForce RTX 3090 (24 GB), Vulkan, 4K, 6 frames, wall-clock seconds,
no environment variables. 3 runs each, interleaved base/memcap; min / median:

comp	build	in flight	min	median	OOM	output
d_blurglow	base 13a49c9	2	44.75	45.77	0	DEA9290B5BAC
d_blurglow	base 13a49c9	3	(earlier run) 72.94	–	yes	931521E85733 (CPU output: frames that ran out of memory fell back to the CPU path)
d_blurglow	memcap	2	43.86	45.15	0	DEA9290B5BAC
d_blurglow	memcap	3	40.75	44.25	0	DEA9290B5BAC
d_blurglow	memcap	4	41.88	43.30	0	DEA9290B5BAC
c_effects	base 13a49c9	2	40.39	45.53	0	BC9186ED92A5
c_effects	memcap	2	42.92	45.56	0	BC9186ED92A5
c_effects	memcap	3	41.84	45.21	0	BC9186ED92A5
c_effects	memcap	4	41.27	41.41	0	BC9186ED92A5

Correction: All memcap runs match the base's GPU output (DEA9290B5BAC), so none fell back to the CPU. 931521E85733 is the CPU-path output. Plain main's GPU export only matches it because main runs out of memory and silently re-renders on the CPU (the #879 problem). The base at 3 in flight produced it the same way, not because the output was corrupted. DEA9290B5BAC is the genuine GPU output and is within 1/255 of the CPU output on ~0.01% of pixels (PSNR 93 dB).

• At 2 in flight there's no measurable cost; base itself ranges from 40 to 50 s run to run.
• At 3–4 in flight there are no OOM errors and the output is identical. With a 10000 MiB cap on
  the same card, an earlier session measured 3–4 in flight at 36–38 s median; the 8 GiB default
  trades some of that for headroom on smaller cards.

Tests: cargo test -p effectcraft-gpu (EFFECTCRAFT_REQUIRE_GPU=1) on Apple M4: 212 passed, 1
failed (tests_init::opengl_adapters_get_cpu_compositing_at_once: no GL adapter on macOS; it
also fails on base). Metal has no allocator report, so the gate is a no-op there. New unit test
memcap::tests::defaults_and_cadence.

Summary by CodeRabbit

• Bug Fixes
  • GPU resources are cleaned up when frames and exports finish, including after rendering errors or early cancellation.
  • GPU memory pressure is better controlled during rendering and exports, while resources needed by active work remain available.
• Performance
  • Increased reuse of GPU resources across frames and repeated effects, reducing unnecessary allocations.
  • Unused resources are released over time, while resources involved in in-progress GPU work are protected.

[skylartaylor]

Thanks, the trace in the description is useful. Some things to sort out first:

1. This branch still carries 13a49c94, the per-owner give_back (crates/gpu/src/context.rs:227) that GPU: free effect working textures after each call instead of at frame end #1523 has since replaced with the shared 256 MB pool budget. Please rebase onto the current GPU: free effect working textures after each call instead of at frame end #1523 head and re-run the numbers.
2. mem_gate compares total_allocated_bytes, which includes the upload cache (1536 MB by default), pooled textures and live frames, none of which a wait can free. With the 2 GiB integrated default, a project could sit over the cap and drain the queue on every 8th call without freeing anything. Could the gate check what a wait can actually free, and add some hysteresis? Numbers from an integrated GPU would help a lot.
3. The wait is queue-wide with a 10 s timeout, and poll_readbacks retires the context on any poll error. If a timeout comes back as Err, a slow GPU loses GPU rendering for the rest of the session. Could the gate wait on this Enc's own submission index and treat a timeout as "carry on"?
4. The 8 GiB discrete default never fires on 4–8 GB cards. Some VRAM estimate, or a lower default with the env override for large cards, seems needed.
5. Since Metal has no allocator report, the M4 run doesn't exercise the gate. Could you add an over-cap test (a small cap via EFFECTCRAFT_GPU_MEMCAP_MB) that runs on Vulkan or DX12 CI?

[MatejKozlovsky]

Thanks @skylartaylor for the review! All five points make sense, especially gating only on reclaimable memory with hysteresis and waiting on the encoder's own submission index without retiring the context on a timeout.

I'm tied up with other work for a bit, so I'll rebase onto the current #1523 head, rework the gate and defaults, add the small-cap test, and follow up here with updated numbers as soon as I have time.

[MatejKozlovsky]

Rebased onto the current #1523 head (61acfdcb) and reworked all five points (b40d8fe9):

1. Rebased onto #1523: Dropped the old first-round #1523 base commit (13a49c94) and stacked directly on the current #1523 branch (61acfdcb, shared 256 MB pool budget + stale-only eviction).
2. Gate on reclaimable memory with hysteresis (MemCap::should_wait, GpuContext::kept_bytes):
   • GpuContext::kept_bytes sums the bytes that submitting and waiting on the current Enc cannot free: the upload cache (uploads.bytes), the texture pool's kept set after flushing this owner (Pool::kept_after_flush: live Leases, other owners' in-flight working sets, released textures still guarded by other open encoders, and up to budget on the shared free list), readback staging buffers, and cached zeros textures.
   • MemCap::should_wait(allocated, kept) subtracts kept from total_allocated_bytes and only triggers a wait when allocated > cap and reclaimable >= floor + hysteresis (1/8 of the cap, clamped to 1 MiB..=256 MiB). After a wait, record_wait sets floor to the post-wait reclaimable bytes (and lowers floor whenever reclaimable drops), so an over-cap upload cache or textures held by another open encoder never cause the gate to spin on every check.
   • Enc::mem_gate also calls Pool::flush_owned(self.owner) after self.submit() so excess working textures returned to Pool::owned during a long chain or frame are moved through make_free and dropped before poll_wait.
3. Wait on this Enc's own submission index; carry on on timeout:
   • Enc::submit records last_submit: Option<wgpu::SubmissionIndex> from queue.submit(...).
   • Enc::mem_gate waits via GpuContext::poll_wait(idx, timeout) (serialized under poll_lock), which calls device.poll(PollType::Wait { submission_index: Some(idx), timeout: Some(timeout) }) directly and never calls poll_readbacks / readbacks.retire, so a slow submission or timeout simply moves on without retiring the GPU context.
4. Lower discrete default for 4–8 GB cards:
   • Lowered default_cap(DeviceType::DiscreteGpu) from 8 GiB to 3 GiB (2 GiB for integrated/software adapters), leaving headroom above the 1.5 GiB upload cache + 256 MB pool while firing before 4–8 GB cards hit OOM (EFFECTCRAFT_GPU_MEMCAP_MB still overrides it, and caps smaller than 4 × POOL_BUDGET scale the initial pool budget to cap / 4).
5. Over-cap test on Vulkan/DX12 (tests::mem_gate_reclaims_over_cap_and_tolerates_timeout):
   • Spawns the test binary in a child process with EFFECTCRAFT_GPU_MEMCAP_MB=8 (no unsafe { env::set_var }) and verifies on any backend with an allocator report that: (a) live uploads over the 8 MiB cap do not trigger waits, (b) dropped working textures over the cap trigger mem_gate waits and reduce allocated_bytes(), and (c) a zero-timeout wait carries on without retiring the context.

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Walkthrough

The GPU context now bounds and ages its texture pool, tracks encoder use, gates work based on allocator memory, and serializes polling. Accelerator lifecycle hooks mark frame and export completion across rendering paths.

Changes: GPU memory management and lifecycle

Layer / File(s)	Summary
Bounded texture pool and encoder-safe reuse crates/gpu/src/context.rs, crates/gpu/src/adv3d.rs, crates/gpu/src/lib.rs, crates/gpu/src/tests.rs	The GPU context adds budgeted, frame-aged pooling and tracks leases and encoder use. GPU operations register pooled images before recording work. Tests cover reuse, eviction, retention, concurrency, and encoder safety.
Memory-cap gating and serialized polling crates/gpu/src/memcap.rs, crates/gpu/src/context.rs, crates/gpu/src/effects.rs, crates/gpu/src/walk.rs, crates/gpu/src/tests.rs	The GPU context tracks allocator usage, applies reclaimable-memory hysteresis, serializes polling, and uses sliced waits. Effect and layer rendering paths call the memory gate. Tests cover caps, hysteresis, waits, and polling.
Frame and render lifecycle integration crates/render/src/accel.rs, crates/gpu/src/lib.rs, crates/render/src/passes.rs, crates/ui-egui/src/frames.rs, crates/engine/src/background/remote.rs, crates/export/src/lib.rs, crates/export/src/pipeline.rs, crates/export/tests/it/h264_gpu_yuv.rs	The accelerator interface adds frame-end and render-end hooks. Rendering paths call frame-end, and export cleanup calls render-end. The GPU accelerator connects the hooks to pool maintenance.

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Enc
  participant MemCap
  participant GpuContext
  participant Queue
  Enc->>MemCap: check allocator usage and reclaimable bytes
  MemCap->>GpuContext: request wait when gate threshold is reached
  GpuContext->>Queue: submit pending work
  GpuContext->>Queue: wait for submitted work
  Queue-->>GpuContext: completion or timeout
  GpuContext-->>Enc: record memory and retained-byte counts

Loading

Possibly related issues

• App crush when playback is going #2298: The memory gate waits for GPU work when allocator usage and reclaimable memory meet its thresholds; the issue describes a GPU device loss during command submission from memory pressure.
• GPU export: render --gpu sends the GPU a whole batch of frames, runs out of memory and falls back to the CPU without logging it #897: The memory gate controls GPU work based on allocator usage; the issue describes concurrent export frames exhausting GPU memory. Its CPU-fallback logging request is not addressed here.
• GPU Not using #2423: The memory gate adds allocator-based waits during GPU work; the issue reports Vulkan out-of-memory errors during rendering.
• [Bug Report] [EffectCraft] **Subject: GPU Memory Usage Causing System Crashes During Export** #1487: The memory gate adds a cap and submission waits for GPU memory; the issue requests adaptive limits on frames in flight during export.

Suggested reviewers: echelon, bflatastic

Merge Risk: Minimal · previous review 44956

No actionable merge-blocking issue is established; the UI rendering paths end each queued frame at most once.

Security architecture risk: Low · up to 44956

The inspected paths preserve active resources and submission ordering. Remaining risk is primarily availability under memory pressure: the new controls are advisory and do not guarantee a memory ceiling for every workload or platform.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The relevant availability domain is concurrent rendering sharing a GPU context or device. The free-list budget excludes live owners and guarded releases, and the gate deliberately skips memory that waiting cannot reclaim; these controls are not a per-frame or tenant memory quota.

Trust Boundaries and Controls

• observed — The cap override comes from the process environment at context construction, not from a frame parameter. Parsing accepts whole unsigned MiB with checked multiplication; invalid or overflowing values fall back to the device-type default.

Resilience and Maintainability Implications

• observed — Device polling is serialized. Nonblocking polls skip a contended mutex, while blocking maintenance uses poll slices of at most 20 ms. Memory-gate timeouts allow rendering to continue; other poll errors retire readbacks, and native readback deadline expiry also retires the context.

Hardening Proposals

• proposed — Consider narrowing raw encoder access or adding typed recording helpers so future callers cannot accidentally omit pooled-image escape tracking. The current contract explicitly relies on caller convention; this is a future control-drift precaution, not an observed bypass.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding GPU memory back-pressure to reduce out-of-memory errors when multiple frames are in flight.
Docstring Coverage		Docstring coverage is inconclusive: files from earlier reviews still need to be rechecked within the check's analysis limits. Not blocking.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @crates/gpu/src/context.rs:
- Around line 967-971: Update the `poll_lock` handling in `Gpu::poll` so
`PollType::Poll` never waits for the lock, while blocking waits remain
serialized. Adjust the blocking readback path around `poll_wait` and
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Actionable comments posted: 1

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Review comments at @crates/gpu/src/context.rs:
- Around line 998-1004: Update poll_wait’s device.poll result handling so
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[bflatastic]

Thanks @MatejKozlovsky! After #1631 this no longer compiles: it makes Enc::encoder() private, but gpu/src/yuv.rs calls e.encoder(), and the test-only without_kernel needs the new poll_lock and memcap fields. Please rebase and keep encoder() pub(crate) (or switch yuv.rs to e.record(..)). Since this contains all of #1523, I'll treat #1523 as included here, with #1706 (the sysfs VRAM cap) to follow on top.

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Actionable comments posted: 1

Nitpick comments: 2

crates/gpu/src/context.rs: 1

1739-1739: Trivial · Maintainability & Code Quality · Quick win

Register src in Enc::crop through record.

This change makes Enc::record and Enc::uses the way to mark foreign leases as escaped. The record doc says that skipping this step reintroduces the reuse race (#1441). copy_into now calls self.record(&[src, dst]). crop still records copy_texture_to_texture from src through self.encoder() and does not mark src.

If src belongs to another Enc, its lease is not marked escaped. When that lease drops, Lease::drop returns the texture through release_owned to its owner. The owner can then overwrite the texture while this encoder's copy is still unsubmitted.

Current callers pass images that this Enc made, for example draw_adjustment. The race is therefore not reachable today. The fix is one line and makes crop consistent with copy_into.

♻️ Proposed change

-        let enc = self.encoder();
+        let enc = self.record(&[src, &dst]);
         enc.copy_texture_to_texture(

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Review comment at @crates/gpu/src/context.rs at line 1739:
Update Enc::crop to obtain its encoder through Enc::record, registering both src
and the destination texture before issuing copy_texture_to_texture. Keep the
copy operation otherwise unchanged so foreign source leases are marked as
escaped.

crates/gpu/src/tests.rs: 1

1462-1466: Trivial · Maintainability & Code Quality · Quick win

Honor EFFECTCRAFT_REQUIRE_GPU in the memory-cap child test.

The child process calls Gpu::headless() directly. If it gets None, it returns without an error. This bypasses require_gpu, so EFFECTCRAFT_REQUIRE_GPU=1 does not fail the test when GPU initialization fails.

The skip at allocated_bytes() == 0 is also silent. A Vulkan or DX12 CI job that loses its adapter therefore reports this test as passed. The over-cap path then never runs. The PR review asked for this test specifically for Vulkan/DX12 CI.

Route the child through require_gpu. Also print a line on the allocator-report skip so the skip shows in the parent's captured output.

♻️ Proposed change

-    let Some(g) = Gpu::headless() else { return };
+    let g = Gpu::try_headless();
+    let Some(g) = require_gpu("mem_gate child", g.as_ref()) else { return };
     let ctx = g.context();
     if ctx.allocated_bytes() == 0 {
+        eprintln!("mem_gate child: no allocator report on {}, skipping", ctx.name);
         return; // backend has no allocator report (Metal, GL, WebGPU)
     }

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Review comment at @crates/gpu/src/tests.rs around lines 1462 - 1466:
Update the memory-cap child test to initialize the GPU through the existing
require_gpu helper so EFFECTCRAFT_REQUIRE_GPU=1 fails when initialization fails,
and print a skip message when ctx.allocated_bytes() is zero so the parent
captures the allocator-report skip.

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Inline comments:
Review comments at @crates/gpu/src/memcap.rs:
- Line 45: Update the GPU memory-cap calculation so an absent
EFFECTCRAFT_GPU_MEMCAP_MB uses default_cap(device_type), while an unparsable or
overflowing value logs a warning and falls back to that default. Replace the
shifting conversion with checked multiplication by the MiB scale so large values
cannot wrap.

---

Nitpick comments:
Review comments at @crates/gpu/src/context.rs:
- Line 1739: Update Enc::crop to obtain its encoder through Enc::record,
registering both src and the destination texture before issuing
copy_texture_to_texture. Keep the copy operation otherwise unchanged so foreign
source leases are marked as escaped.

Review comments at @crates/gpu/src/tests.rs:
- Around line 1462-1466: Update the memory-cap child test to initialize the GPU
through the existing require_gpu helper so EFFECTCRAFT_REQUIRE_GPU=1 fails when
initialization fails, and print a skip message when ctx.allocated_bytes() is
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[MatejKozlovsky]

Thanks @bflatastic! Rebased onto 990fa668, and it builds again. Enc::encoder() stays pub(crate) for yuv.rs, and its doc now says when skipping Enc::uses is safe. The test-only without_kernel shares poll_lock (now an Arc, since it's the same device) and gets a MemCap with the same settings but its own counters.

While rebasing I found that the GPU H.264 export path from #1631 never called frame_end, so the texture pool never trimmed during exports. pipeline.rs now calls it after comp_frame_yuv420, and h264_gpu_yuv.rs checks that each frame ends exactly once.

44956ef1 addresses CodeRabbit's review. An invalid or overflowing EFFECTCRAFT_GPU_MEMCAP_MB now warns and falls back to the default (0 still disables the gate). Enc::crop registers its source through Enc::record. The mem-cap child test now goes through require_gpu and reports its skips.

fmt, clippy, wasm32 and the tests pass locally, except 4 font-listing tests that fail the same way on current main because of a system font on my machine.

[bflatastic]

Thanks @MatejKozlovsky, the rebase looks good on reading. One thing held it back: merged with current main, your own tests::stale_sizes_do_not_starve_a_steady_size fails on a real GPU (steady 300x300 calls after stale sizes: 0 hits, 20 misses). Main's Lease now carries the discard generation from #1319, and a lease whose generation changed is dropped instead of returned to the pool, so with your back-pressure accounting the steady size never gets a hit. Could you merge main, reconcile Lease::drop with the generation check (keeping both your escaped/leased accounting and main's discard), and run cargo test -p effectcraft-gpu --lib on a machine with a GPU? CI has none, so it skips these tests.

[MatejKozlovsky]

Thanks for running it on real hardware. You were right: stale_sizes_do_not_starve_a_steady_size fails. I reproduced it on an Apple M4 Max (Metal), and got the same "0 hits, 20 misses".

Root cause. It isn't the Lease::drop / generation interaction with #1326. The test fails the same way on the original, unrebased PR commits, so the bug was already in the PR before the merge. The generation counter only changes in Pool::discard, i.e. on a failed frame, and nothing in the eviction path bumps it or calls discard.

The real problem is Pool::dying. Evicted textures stay counted there until the queue's work-done callback fires. At frame end, pool_frame_end submitted an empty batch and then did a non-blocking Poll. On Metal that poll usually returns before the callback runs, so dying never drained. Once dying + evicted + size > budget, make_free stopped evicting stale sizes and dropped the steady size every frame. That is where the 20 misses came from.

Fix (in commit 4, "evict only stale pool sizes"). When dying is non-zero, pool_frame_end now submits an empty batch and waits on that submission's index, for at most the readback timeout. Otherwise it keeps the cheap Poll. A wait timeout just moves on. It does not retire the context or its readbacks (the plain poll_readbacks path would have), so a slow GPU only delays eviction and doesn't kill the preview. The pool lock is released before the submit and the wait, and the callback only touches an atomic. In the memory-cap commit the wait becomes the same poll_wait the cap uses. It runs in 20 ms slices under poll_lock, so a readback on another thread never waits behind a whole timeout, and concurrent device.poll calls stay serialized.

Lease::drop and generation, after the rebase onto current main:

• A lease always subtracts its bytes from leased first. That includes a stale-generation lease from a discarded failed frame, because Pool::discard doesn't reset leased.
• Then the GPU: contain frame-local validation failures (refs #1319) #1326 check runs: if the generation doesn't match, the texture never goes back to the pool, escaped or not.
• Only then do we use the guarded release (escaped) or release_owned.
• pooled takes the texture, adds the same bytes to leased and reads the generation, all under one lock.

Rebased onto current main (ec7ced1, includes #701). The rebase was clean. The rebase review turned up two fixes, both squashed into their commits. poll_wait now uses web_time::Instant, because pool_frame_end reaches it on wasm32, where std::time::Instant::now() panics. And yuv420_frame now ends the frame only after main's new plane-length check, so a mismatch that falls back to passes::comp_frame doesn't end the same frame twice.

Results on the rebased head:

• Apple M4 Max (Metal), EFFECTCRAFT_REQUIRE_GPU=1 cargo test -p effectcraft-gpu --lib: 261 passed, 1 failed, 0 ignored. The failure is opengl_adapters_get_cpu_compositing_at_once, which fails the same way on main on this Mac. stale_sizes_do_not_starve_a_steady_size gets 18 hits, 2 misses and 28 evictions (printed with a temporary eprintln). The 2 misses are the cold first uses.
• Software adapter (lavapipe, Vulkan, Linux): cargo test -p effectcraft-gpu --lib 262 passed, 0 failed. With EFFECTCRAFT_REQUIRE_GPU=1, stale_sizes_do_not_starve_a_steady_size also gets 18 hits, 2 misses and 28 evictions.
• fmt, clippy -D warnings, the wasm32 check and cargo xtask ci pass, apart from two font tests that also fail on main in that environment.

Could you re-run it on your 4090?

[MatejKozlovsky]

Pushed dbedf69: after a pool frame-end wait that frees none of the evicted bytes, later frames only poll instead of re-waiting up to 10 s each (same change as on #1523).

[FredApps]

ReviewerA — source-read COMMENT (not an approval)

I read the first hunk: in the remote frame server, after a frame is done (frame_begin ran in handle), a.frame_end() is called so the GPU pool ages its sizes; per the title the GPU gets memory back-pressure for frames in flight, to fix out-of-memory at three or four frames in flight. A limit on frames in flight by memory (not by count) is the right control: a frame must wait until the pool can hold it, the wait must be bounded or cancellable (a frame that can never fit must fail with a clear error, not wait for ever), and every exit path of handle (an error, a cancelled frame) must still call frame_end, or the pool never ages and the back-pressure deadlocks. The first hunk calls it only after a normal pick: please check the error returns. I did not read the pool. Add tests for a frame bigger than the budget, two frames that fit one at a time (the second waits and then runs), and a frame that fails (the budget is released). Nothing was run.

I ran nothing and have no exact-head CI result for this PR, so this is not an approval. Where a diff was long I read its code hunks rather than every generated or documentation line.

Status: waiting_author.

[MatejKozlovsky]

Thanks for the careful look. On frames that can never fit: I didn't make that a hard error, because the cap is back-pressure on memory we can get back, not admission control, and failing big comps outright would be a regression. Every wait is bounded by the readback timeout, so nothing waits forever. And when the memory held by live frames, the upload cache and the pool is over the cap on its own, the gate logs a one-time warning that names the env var and doesn't wait for memory it can't free. It still waits when enough of the rest can be reclaimed. If you'd still rather have a hard error in that case, I can add one.

On frame_end: the remote server's early fails all return before frame_begin, so there's nothing to end there. The real gap was a render arriving while a multi-pass frame was still pending. That frame is now ended before the new one begins. passes::comp_frame ends the frame through a drop guard, so it also runs on a panic. Since each frame worker takes one frame at a time and runs it pass by pass, the replaced-pending case is mostly about keeping the API invariant, but now it holds.

Tests: every_frame_begin_has_a_frame_end (remote), comp_frame_ends_the_frame_when_the_render_panics (render), a memcap unit test for the warn-once/no-wait case, and new sections in mem_gate_reclaims_over_cap_and_tolerates_timeout: a frame over the cap, two frames that fit one at a time (the second waits for the first's memory, then reads back), and a dropped frame giving its memory back so the next one doesn't wait.