Recover independent-context object-churn scaling

[chrisbbreuer]

Problem

The clean July 22 comparison (0c9b329a, zig-gc 88ea2543) exposes a severe object-churn scaling failure while the other nine workloads still beat JSC at eight warmed contexts. zig-js moves from 114.636 ms at one warmed lane to 3,548.104 ms at eight lanes (0.26x throughput scaling); system JSC moves from 122.697 ms to 195.609 ms (5.02x). The cold eight-lane row is similarly 3,438.036 ms versus 198.845 ms.

This is not directly comparable to the July 17 dependency/toolchain state, but it is the current user-visible bottleneck and dominates the aggregate regression.

Acceptance

• Reproduce with the saved raw matrix and the focused object-churn profiler.
• Attribute allocator, GC rendezvous, nursery, publication, and worker lifecycle time at 1/2/4/8 lanes.
• Land a causal exact-parent A/B with checksum and retained-memory parity.
• Restore monotonic independent-context scaling without regressing shared-realm safety or other matrix workloads.
• Run focused normal/TSan gates, then regenerate the complete comparison and README scorecard.

[chrisbbreuer]

Baseline evidence is now committed in 69c975e: the July 22 report and all 1,540 raw samples are linked from the README and docs. The current code baseline also passes 1,404/1,404 tests with no failures or leaks. Keep this issue open: the next batch should begin with a focused 1/2/4/8-lane profiler that separates allocator locks, GC rendezvous, nursery collection, slab publication, and worker lifecycle before changing code.