feat(rust): port compression block pre-splitting

Move ZSTD_splitBlock into Rust while retaining its caller-owned workspace
contract. The implementation preserves the C fingerprint sampling heuristic
and calls the migrated histogram primitive without adding a hot-path
allocation.

Reference-vector and randomized differential tests compare every split level
with the original C translation unit. The C source remains as a declaration
shim so existing source lists resolve the Rust ABI during the migration.

Test Plan:
- cargo fmt, cargo clippy, cargo clippy --benches, cargo clippy --tests
- cargo test --all-targets and cargo build --release
- cargo test/build --target i686-unknown-linux-gnu
- 264-case original-C/Rust differential harness
- C fuzzer, zstreamtest, and fuzzer32 smoke runs

Refs: rust/README.md

rust/README.md

@@ -26,6 +26,7 @@ zstd ABI:
     Huffman streams.
 - Compression primitives
   - `hist` counts byte frequencies for FSE and Huffman compression.
+  - `zstd_presplit` chooses split points for full compression blocks.
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.