Move the serial turn wait loop out of zstdmt_compress.c and into a Rust
entry point. Rust now owns the lock, condition-wait, and ready/skip ordering
policy while C retains pthread synchronization behind lock and wait callbacks.
The projection documents that the mutex remains locked on return for the
serial advance callback, preserving the existing worker synchronization
contract. Add ABI layout checks and focused tests for both waiting and skip
paths.
Test Plan:
- cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml zstdmt_compress::tests::serial_wait_for_turn -- --nocapture
- ulimit -v 41943040 && make -j1
ddidderr
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