Test: tolerate conservative ArrayBuffer recovery roots under TSan

Related: #50, #100

Failure

CI run https://github.com/zig-utils/zig-js/actions/runs/29449252934 failed only tsan-unit (shard 1/4) at enable_gc: heap_limit_bytes retries ArrayBuffer byte allocation after collection: after the successful retry, gc_array_buffer_bytes_live was 40 MiB instead of the test-exclusive 20 MiB. The same full TSan suite passed locally before publication.

Root cause

The allocation-failure path intentionally performs a conservative native-stack scan because arbitrary interpreter/Zig temporaries may hold managed values. A stale word in the newer CI Zig/TSan stack layout can conservatively retain the first unreachable ArrayBuffer wrapper for that one recovery collection. This is permitted collector behavior, not a leak: the second allocation still demonstrates that collection freed enough budget and the retry succeeded, while the subsequent quiescent precise collection must release every unreachable buffer exactly.

Scope / acceptance

• Accept either one or two live 20 MiB buffers immediately after the successful recovery allocation; reject any other accounting value.
• Keep the final post-drop quiescent collection assertion exact at the original baseline.
• Preserve the initial below-allocation budget witness, successful retry, exact result, and separate exact ArrayBuffer-release test.
• Pass the focused normal/TSan test and the next batched CI run.

Implemented and pushed as de0a781b (test(gc): allow conservative recovery roots (#121)). The intermediate assertion now accepts exactly one or two live 20 MiB buffers after the conservative recovery cycle; the post-drop quiescent collection still requires the exact original baseline. Focused gates are green: normal 2/2 and TSan 2/2, with 0 failures, leaks, or sanitizer reports. Keeping this open until the new full CI run completes.

Acceptance is complete. Focused normal/TSan coverage was green before publication, and combined main run 29462319298 completed every normal unit shard plus the exact tsan-unit shard 1 successfully at d35841a before a later Date-only follow-up superseded the run. The conservative recovery assertion remains bounded to exactly one or two live 20 MiB buffers, with the final quiescent baseline exact. Closing.