Complete the single-thread branch of ZSTD_getFrameProgression behind a
narrow scalar ABI boundary. C still owns the public entry point, buffered
input extraction, assertions, and the multithreaded dispatch; Rust now owns
the six-field single-thread result construction. The Rust #[repr(C)] result
uses wrapping u64 conversion for consumed input plus buffered size, mirrors
produced into flushed, and zeros the MT-only fields.
This replaces the earlier ingested-only helper so the ABI no longer splits
one result across duplicate C and Rust construction paths. The unit tests now
cover the complete result and its u64 wrapping behavior.
Test Plan:
- All three requested compression clippy commands passed before and after
`cargo +nightly fmt --manifest-path rust/Cargo.toml`.
- `cargo test --manifest-path rust/Cargo.toml --no-default-features
--features compression` -- 327 passed.
- `make -B -C lib -j2 lib` -- passed.
- `make -C tests test-rust-lib-smoke` -- passed.
- `tests/fuzzer -s4560 -t56 -i57 -v` -- passed.
- `make -C tests -j2 test-zstream` -- passed: 84 named, 5,330, and 9,772
randomized cases.
- `git diff --check` and `git diff --cached --check` -- passed.
The zstream build retains the pre-existing warning at
`tests/zstreamtest.c:1899` about an unterminated initializer string.
ddidderr
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