feat(compress): move block split search to Rust

Port the recursive post-block-split partition search behind a narrow Rust ABI leaf while keeping CCtx ownership, entropy-state projection, and block emission in C. Preserve left-first midpoint insertion, minimum-sequence gating, the split cap, and error-as-no-split behavior.

Test Plan:

- cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression

- cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression --all-targets

- cargo +nightly fmt --manifest-path rust/Cargo.toml

- make -B -C lib -j2 lib

- tests/fuzzer -s4560 -t47 -i48 -v

rust/src/lib.rs

@@ -37,6 +37,8 @@ pub mod zstd_compress;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_api;
 #[cfg(feature = "compression")]
+pub mod zstd_compress_block_split;
+#[cfg(feature = "compression")]
 pub mod zstd_compress_dictionary;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_frame;