Perf: use the exact-managed barrier in object churn replacement

[chrisbbreuer]

Problem

The fixed object-allocation loop replaces a present dense array element with a freshly allocated managed object on every iteration. Its generic replaceDenseElement barrier classifies both exact GC payload pointers through zig-gc’s tolerant live-payload index, which remains visible in the object_churn profile.

Scope

• Add a narrow exact-managed value barrier in gc.zig backed by zig-gc#10.
• Add an explicitly prevalidated, exclusive dense replacement primitive that omits its own generic barrier.
• Use that pair only in the isolated object-allocation quick loop when the active allocator is the GC heap; preserve the existing arena/generic path.
• Keep the per-iteration restriction check and all loop bailout semantics unchanged.
• Add focused coverage proving the quick loop still runs correctly in both arena and GC modes, including nursery collection.
• Compare pinned alternating runs against cfed7bc5; land only if the exact-checksum benchmark shows a repeatable win.
• Run the batched full suite only after a measured win.

Acceptance

• Exact managed owner/child barrier preserves nursery and incremental invariants.
• object_churn checksum remains 1030552128.
• Candidate beats the pinned baseline across paired samples.
• Commit references this issue and is pushed to main as Chris without coauthors.

[chrisbbreuer]

Landed on main in fb6f8d8, backed by zig-gc exact-managed barrier b9b5a8c (zig-gc#10). Seven alternating process pairs with three samples each beat cfed7bc in 7/7 pairs: overall median 254.840333 ms -> 232.842708 ms (8.6319% lower, 1.0945x throughput), with checksum 1030552128 in all 42 samples. Validation: focused GC/nursery quick-loop coverage, 757/757 unit tests with zero leaks, and 7/7 benchmark-harness tests.