Move the worker-count branch of ZSTD_toFlushNow into the Rust policy layer.
The C adapter retains the private CCtx and multithread context, exposing only
the worker count and a callback for the MT flush query. Single-threaded
contexts keep the existing zero result and the ABI projection is checked on
both sides.
Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml to_flush_now --lib
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1
ddidderr
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