feat(compress): move single-block serialization into Rust
`ZSTD_compressSeqStore_singleBlock` was still a C-owned decision body after
sequence encoding and block-format leaves had moved into Rust. That left the
C function responsible for mixing entropy errors, partition repcode repair,
RLE/raw fallback, sequence collection, compressed-state confirmation, and
block-header emission through the full private compression context.
Keep sequence-store construction, matchfinding, and split-block orchestration
in C, but pass a narrow repr(C) projection to Rust. The Rust body preserves the
original order of partition off-code resolution, entropy compression, the
non-first-block RLE rule, collector handling, dRep restoration for raw/RLE
blocks, compressed-state confirmation, header serialization, and repeat-mode
cleanup. Injectable leaf seams and focused tests cover entropy errors, raw
fallback, RLE gating, pointer swaps, and repeat-mode transitions.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --lib -- --test-threads=1` -- 447 passed
- `cargo clippy --manifest-path rust/Cargo.toml -- -D warnings` -- passed
- `cargo clippy --manifest-path rust/Cargo.toml --benches -- -D warnings` -- passed
- `cargo clippy --manifest-path rust/Cargo.toml --tests -- -D warnings` -- blocked by the pre-existing manual_repeat_n lint in one_shot_promotes_nonfirst_rle_blocks
- `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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