The public ZSTD_compressStream2_c entry point still contained the complete
fallback state machine even though its validation, initialization policy,
serial-versus-MT dispatch, and result accounting were already represented by
Rust policy helpers. That left the main streaming entry point as a large C
orchestration island and made the Rust rewrite boundary misleading.
Project the scalar stream state and private CCtx operations through a checked
C ABI, then let Rust own the fallback ordering. Rust now validates buffers and
the end directive, handles transparent one-shot completion and stable-input
deferral, performs the initialization and stability decisions, selects the
serial or MT path, and publishes the result policy. C retains the private
context layout, stream adapter, MT operations, checksum/epilogue side effects,
and trace/reset callbacks. ABI offset assertions and focused callback-order
and stable-input tests protect the projection without requiring a standalone
Rust test binary to link every C bridge symbol.
Test Plan:
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo +nightly fmt --manifest-path rust/Cargo.toml --all` -- passed.
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings` -- passed before and after formatting.
- `git diff --cached --check` -- passed.
- Native `make -j1` and the original suite remain the next post-commit verification gates.
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