feat(decompress): move decoder allocation bridges into Rust
Decoder orchestration already crossed into Rust, but the C translation unit
still owned the allocation bridge used by every decoder context and dictionary
set. That split kept custom malloc/calloc/free behavior, allocator-pair
validation, and the decoder-context storage lifecycle in C without focused
Rust coverage.
Move those bridges into Rust while keeping the C-defined `ZSTD_DCtx` layout and
configuration-dependent platform, legacy, and trace leaves in C. Rust now
validates custom allocator pairs, allocates and frees the opaque decoder
storage, dispatches default allocations to libc, and preserves calloc's
zero-fill contract for custom allocators. Focused tests cover default memory,
custom callbacks, zeroing, null-safe free, and invalid callback pairs.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --lib -- --test-threads=1` -- 447 passed
- `cargo test --manifest-path rust/Cargo.toml --lib zstd_decompress -- --test-threads=1` -- 6 passed
- `cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -B -C tests -j2 test-cli-tests` -- passed
- `ZSTREAM_TESTTIME=-T2s make -B -C tests -j2 test-zstream` -- passed
- `FUZZERTEST=-T5s make -B -C tests -j2 test-fuzzer` -- passed
- `make -B -C tests/fuzz -j2 all` and `sequence_compression_api` -- passed
- `make -C tests -j2 test-zstd` -- passed, including large-data cases
ddidderr
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