Keep ZSTD_CStream context access and caller-specific streaming behavior in C,
while moving the pure stable-versus-buffered input-buffer policy to a scalar
Rust ABI helper. The helper returns the public repr(C) buffer layout, preserves
stable-mode source, size, and position values including NULL sources, and
returns a zeroed buffer for every other mode.
Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed before and after formatting
- the same clippy command with `--benches` and `--tests` -- passed before and after formatting
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- 274 passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests test-rust-lib-smoke` -- passed
- `make -B -C tests -j2 fuzzer` and `tests/fuzzer -s4560 -t56 -i57 -v` -- passed
- `make -C tests -j2 test-zstream` -- 84 named tests and 7,400 plus 8,519 randomized cases passed
- `git diff --check` and `git diff --cached --check` -- passed
ddidderr
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