Add reusable item virtualization and recycling foundation

[ProGraMajster]

Summary

Add the shared virtualization foundation needed before #55 control integration and #13 collection controls. Indexed layouts plan a viewport/cache range without enumerating logical values; the typed generator realizes that range and recycles containers with explicit ownership. Parent #55 remains open for #200.

Architecture

VirtualizingLayout extends the existing LayoutEngine through IVirtualizingLayoutHost. Vertical/horizontal stack and fixed-cell wrap geometry use logical pixels. A measured prefix tree supports variable stack extents and unrealized-item offsets. ItemIndexChange maps occurrences through indexed source mutations, keeping equal values distinct.

Public API

Ten additive types in ModernFormsNext.Virtualization: range/context, layout/host contracts, stack/wrap strategies, metrics, occurrence mapping, and generic generator/factory. Existing APIs are preserved. XML documentation and docs/virtualization.md explain usage, limits and ownership. SDK ApiCompat against exact starting master passes for Debug/Release and both core TFMs, with parameter-name/attribute rules, no suppressions, and fully resolved references.

Lifecycle/threading

Use the host owning thread and existing Control/dispatcher/focus/binding checks. The generator borrows its factory/source, coalesces reentrant range requests, queues mutations in order, and prevents older callbacks from overwriting newer state. Callback disposal is deferred until callback return; fault cleanup attempts every owned container and propagates errors. No finalizer or separate dispatcher is introduced.

Platform behavior

The implementation is shared and introduces no platform-native types. Geometry uses finite logical coordinates; overflowing extents/far edges throw. No backend changes, new dependencies, package version changes or publication.

Designer

These strategy/metadata constructors start no native host. Control property persistence, bounded previews and code-generation acceptance remain required by #200.

Tests

• Solution restore and serial Debug/Release builds: pass, 0 warnings/errors.
• Targeted virtualization: 58/58; 100k+ geometry, 10k scroll/jump passes, bounded pools, warmed zero-allocation assertions, variable measurements, indexed mutations, reentrancy/disposal/failures.
• Full Debug: 4421/4421, 0 skipped across nine projects in the full-context comparison after investigation.
• Full Release: 4421/4421, 0 skipped across nine projects.
• Exact starting master full Debug: 4363/4363.
• Documentation source gate and diff checks: pass. Post-commit DocFX metadata/HTML: 0 warnings/errors; all four local documentation/API/sample/SDK archives validated for commit 9f86cec (existing version, no publication).
• Existing-version core NuGet inspection: DLL hashes match qualified Release outputs for net10.0/net10.0-windows7.0; XML includes all ten new types.

Native validation

Existing Windows native suites run in full validation. This child establishes engine behavior; rendered virtualization, selection/focus/accessibility, Android emulator/physical-device and Designer control integration are required by #200. No new virtualized UI/native performance claim is made here.

Known limitations

Fixed-cell wrap explicitly rejects variable metrics. Metrics retain numeric metadata proportional to logical count; reset is O(count). Hosts own scroll anchors, logical selection/focus and existing integer-scrollbar limits. Async/infinite-source contracts are not implemented.

Two original Debug native failures are preserved in local evidence: a Designer host WS_VISIBLE assertion and modal owner-input release. Neither was reproduced in ten predetermined alternating exact-base/feature trials for each test. The append-only PID log permits an old FORM_SHOWN marker to satisfy the Designer wait; this synchronization hazard is documented as unresolved. No assertions, timeouts, native flags or thresholds were changed. Successful subsequent checks do not mean these failures were fixed.

Issue linking

Fixes #199.
Part of #55. Integration continues in #200 after this PR merges; #55 remains open.