Threads: serialize no-GIL access to embedder allocators

[chrisbbreuer]

Parent: #1
Related: #15, #49

Problem

A preserved pre-rewrite experiment identified a valid invariant gap that remains in the current allocator architecture. Under parallel_gc, arena chunk refills are serialized by LockedArena and GC slab refills are serialized by GcCellBacking, but those are independent locks and both ultimately call the embedder-provided std.mem.Allocator. Zig allocators do not universally promise concurrent safety. Two no-GIL mutators can therefore enter a non-thread-safe embedder allocator concurrently even though each engine subsystem is internally locked.

The stale experiment itself cannot be applied: it targets the removed GC layout and points zig_gc at a deleted marker worktree. This issue tracks a clean adaptation to current main.

Acceptance criteria

• True-parallel contexts serialize every underlying embedder allocator alloc/resize/remap/free across arena, GC backing, JIT ownership, and context side structures.
• Single-threaded and explicit-GIL contexts add no allocator lock.
• Heap-budget accounting and allocation-recovery lock ordering remain correct and deadlock-free.
• Context create/error/teardown paths free the wrapper after every allocator consumer.
• Add a concurrent probe allocator test that fails on overlapping underlying calls and passes through the wrapper.
• Pass focused lifecycle/no-GIL semantic tests and suppression-free TSan.
• Measure allocator contention after the fix; optimize only if Perf: remove no-GIL scaling bottlenecks in the comparison workloads #49 evidence identifies it as material.

[chrisbbreuer]

Implemented and pushed in 35dff22. True-parallel contexts now install one bottom-level serialized allocator beneath the budget allocator and every context consumer; single-threaded/GIL contexts retain the raw allocator path. The wrapper is destroyed last. Validation: zig build test -Doptimize=ReleaseFast -Dtest-filter=SerializedAllocator — 2/2 passed, 0 leaked. TSan and contention measurement remain deliberately open.