The Rust simple-compression entry points already owned the dispatch, but
ZSTD_rust_compressCCtxStrategy still called the C parameter cascade to decide
whether a fast Rust frame path was safe. That left a small policy leaf in the
C implementation and made the dispatch boundary harder to test in isolation.
Implement the helper in Rust by using the existing Rust table-selection and
C-parameter-adjustment functions with the same source-size, dictionary-mode,
and automatic-adjustment inputs as the C code. Leave only the C ABI declaration
and add tests for the table thresholds, default-level behavior, and negative
fast-level behavior. The private context reset and fallback compression paths
remain unchanged.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --lib zstd_compress --
--test-threads=1` -- passed (234 tests).
- `cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed.
- `make -B -C lib -j2 lib` -- passed.
- `make -C tests -j2 test-rust-lib-smoke` -- passed.
- Full-target clippy remains blocked by pre-existing test-only
`manual_repeat_n` and `manual_dangling_ptr` warnings outside this seam.
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