GC: recover bounded heaps for interpreter side-storage allocation

Parent: #134

The bounded-heap object side-store regression test could exhaust its 4 MiB budget with zero collection attempts. Interpreter environments, promise reactions, and ArrayBuffer-owned buffers allocated through GcCellBacking; delegated non-cell allocations held that allocator's internal lock, where recursive allocation recovery is intentionally blocked. Cell slabs now remain on GcCellBacking, while interpreter-owned reclaimable side storage uses the Context budget allocator directly so an allocation miss can collect at a safe outer boundary.

Acceptance

• Route GC-mode interpreter side storage through the Context allocator, separately from GC cell slabs.
• Preserve exact finalization/accounting for environments, promise reactions, and ArrayBuffers.
• Prove the bounded-heap side-store workload performs at least one collection and completes.
• Keep the allocator-internal recovery suppression invariant intact.
• Record the affected Debug/ReleaseSafe fixing CI and close.

Implemented in a423b52f. The formerly failing 4 MiB workload now completes and asserts that at least one collection occurred. Unit shard 1 passes 308/308, and the focused regression passes in Debug and ReleaseSafe with zero leaks. Awaiting fixing main CI.

Completed on main at a423b52f. The exact bounded-heap recovery regression is green in unit shard 1; optimized/concurrent coverage is independently green in releasesafe-threadfuzz and focused-engine-tests. The workload now performs collections instead of reaching OOM with zero recovery attempts.