Memory: enforce JSON stringify heap-budget peak contract in CI

[chrisbbreuer]

Parent: #480. Related: #473, #722.

Current main 8e1e441d2f96867efc40da6cafcc6fbeed0f5476 fails the Linux Debug unit shard 2/4: context.test.heap_limit_bytes: JSON stringify bulk string runs fail atomically, at stats.peak_bytes <= stats.limit_bytes (src/context.zig:12280). Job result: 489 passed, 54 platform skips, 1 failed, 0 leaked; Zig 0.17.0-dev.1818 on the runner. The preceding 2182-pass/1-skip full local macOS Debug run did not reproduce it.

The first JavaScript atomic-failure assertion passed; the failing assertion is the peak-memory contract. BudgetAllocator currently permits a one-time grace after a recovery collection, and reports original limit_bytes separately from grace_granted_bytes. That is a source-level lead, not yet a reproduced root cause.

Acceptance:

• Reproduce with allocator telemetry on the failing platform.
• Reconcile/enforce the documented hard-limit contract without simply raising a cap, weakening the assertion, disabling collection, or hiding the failure.
• Preserve atomic stringify publication and callback ordering.
• Run focused OOM, allocator recovery, no-GIL/TSan and full unit coverage.

Keep #823/#824 open until terminal CI verification is resolved.

[chrisbbreuer]

The Linux TSan unit shard 2/4 also failed the same peak_bytes <= limit_bytes assertion at src/context.zig:12280, not a reported TSan race: https://github.com/zig-utils/zig-js/actions/runs/33219707340/job/99011041278 . Both complete job logs are retained locally. This reinforces that the budget contract needs reconciliation rather than suppressing a sanitizer report.