feat(compress): move ordinary block emission into Rust

Move the post-sequence-store body of ZSTD_compressBlock_internal behind a
small C/Rust state projection. C continues to build the sequence store and
handle the no-compress and sequence-producer error paths, while Rust now owns
sequence collection, entropy emission, the legacy non-first-frame RLE gate,
compressed-block confirmation, and offcode repeat cleanup. Remove the C
wrappers that became dead after those leaves moved behind the Rust body.

The first integration run exposed that leaving the old C finalization label in
place confirmed compressed block state twice, undoing Rust's pointer swap and
breaking a later sparse-file checksum. The C wrapper now returns directly for
the Rust-owned path and retains only its C-owned no-compress cleanup.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib -- --test-threads=1
- cargo check --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings
- cargo clippy --manifest-path rust/Cargo.toml -- -D warnings
- cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check
- make -B -C lib -j2 lib
- make -B -C tests -j2 test-zstd
- make -B -C tests -j2 test-cli-tests
- ZSTREAM_TESTTIME=-T2s make -B -C tests -j2 test-zstream
- FUZZERTEST=-T5s make -B -C tests -j2 test-fuzzer
- make -B -C tests/fuzz -j2 all
- make -B -C tests/fuzz -j2 sequence_compression_api
- cargo clippy --manifest-path rust/Cargo.toml --tests -- -D warnings (pre-existing manual_repeat_n failure)
- cargo clippy --manifest-path rust/Cargo.toml --benches -- -D warnings (pre-existing manual_repeat_n failure)

rust/README.md

@@ -41,7 +41,10 @@ zstd ABI:
   - `zstd_compress_block_split` searches for profitable sequence-store
     partitions, while `zstd_compress` emits those partitions through the
     Rust single-block serializer. C retains split discovery's context setup
-    and the outer block-dispatch decision.
+    and the outer block-dispatch decision. `zstd_compress` also owns ordinary
+    sequence-block entropy emission, sequence collection, and the legacy RLE
+    compatibility gate after C builds the sequence store; C retains the
+    sequence-store builder and no-compress fallback.
   - `zstd_compress_frame` serializes frame headers, skippable frames, and the
     last empty block; it takes scalar frame parameters so the C-owned
     `ZSTD_CCtx_params` layout never crosses the language boundary.