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
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