Move ZSTD_CCtx_refThreadPool init-stage validation and pool-slot assignment
behind a Rust-owned ABI bridge while keeping the opaque pool and CCtx storage
in C. Preserve the stage error and avoid mutating the pool on rejection.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused ref_thread_pool unit tests
ddidderr
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