Normal MT context teardown still let the C wrapper directly sequence
per-job synchronization destruction before Rust released the opaque job-table
storage, while failed create and expansion transactions already used a Rust
helper for that same order. Route the normal teardown through a repr(C)
projection so Rust owns the destroy-before-storage-free policy consistently.
The synchronization callback, descriptor storage, and custom allocator remain
opaque C-owned operations. Add matching C/Rust layout assertions and a focused
callback-order test.
Test Plan:
- `git diff --cached --check` -- passed
- Cargo, make, native tests, and heavy verification were not run per request
ddidderr
10b8b25865