Perf: consume batched zig-gc sweep accounting for object churn

Parent: #62\nRelated: #111\nDependency: zig-utils/zig-gc#23\n\nObject-churn profiles remain dominated by the nursery sweep loop. Consume zig-gc’s batched heap-counter accounting and validate the direct, warmed 1/2/4/8 independent, cold, and shared comparison modes with exact checksums. Acceptance: material stable improvement with no comparison/shared regression; focused and full correctness plus TSan; refresh raw reports, benchmark docs, and README scores; reference this issue from each commit.

Completed by dependency commit zig-utils/zig-gc@9d4af0d and publication commit 86d0c5b. Gates: zig-gc 37/37; zig-js 766/766 normal; TSan 765 passed, 1 expected skip, 0 failed/leaked; full 1,540-sample comparison validated exact checksums and the 50 ms floor. README, benchmark guide, raw TSV, and dated report now carry the latest scores and battery-state caveat.