The MT flush path already delegated checksum serialization to Rust but still
computed XXH64_digest in C. Pass the live C-owned XXH64 state through the
checked projection and let Rust finalize the digest before the existing
little-endian write. C retains serial state storage and job/flush mutation.
Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo check --tests (rust)
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --tests -- -A clippy::manual-bits -D warnings (rust)
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test
- Focused cargo test compiles but remains link-blocked by existing C-owned decompression-view symbols.
ddidderr
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