The explicit-delimiter sequence path still calculated each block summary in C,
with a separate AVX2 implementation and scalar fallback. Move that pure scan
to zstd_compress_stats.rs and keep a C wrapper for the existing orchestration.
The Rust leaf preserves delimiter inclusion, literal and match totals, and the
external-sequences error when no delimiter is present, while the C ABI is pinned
with a repr(C) layout check.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- 227 passed
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` plus benches/tests -- passed before and after formatting
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 test-zstream` -- passed, including 84 deterministic and 15,464 randomized cases
ddidderr
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