Keep the private ZSTD_CCtx and public buffer structures in C while moving the
read-only stable-buffer validation policy behind a scalar ABI. The Rust helper
compares expected and current input pointers and positions without dereferencing
raw pointers, and uses wrapping subtraction for the stable output remainder.
The C wrapper still extracts the context and buffer fields, and the existing
compressStream2 FORWARD_IF_ERROR path remains responsible for propagating the
encoded stability error. The unused endOp parameter is no longer carried into
the validation helper.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed, 309 tests.
- All three requested clippy modes passed before and after `cargo +nightly fmt --manifest-path rust/Cargo.toml`.
- `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 plus 5,333 and 8,072 randomized cases.
- `git diff --check` and `git diff --cached --check` -- passed.
- `make -C tests -j2 test-zstream32` -- unavailable because the i686 Rust target is not installed.
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