ZSTD_selectSequenceCopier combined block-delimiter validation with its
private C function-pointer dispatch. Move only the scalar mode policy behind
a Rust ABI shim, retaining the function-pointer switch and both sequence
transfer implementations in C. Rust preserves the 0..=1 debug validation and
the release fallback to no-block-delimiters for unsupported values.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` — 281 passed.
- Required clippy checks for the library, benches, and tests passed before and after `cargo +nightly fmt`.
- `make -B -C lib -j2 lib` and `make -B -C tests test-rust-lib-smoke` — passed.
- `tests/fuzzer -s4560 -t47 -i48 -v` and `tests/fuzzer -s4560 -t56 -i57 -v` — passed.
- `make -C tests -j2 test-zstream` — 84 named, 7,298 standard randomized, and 9,379 new-API randomized cases passed.
- `git diff --check` and `git diff --cached --check` — passed.
ddidderr
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