feat(rust): port FSE compression primitives

Move FSE normalization, count-header writing, compression-table construction,
and stream encoding from C to Rust. The original C source remains as a
header-only shim, preserving the public header configuration while native
consumers receive the Rust archive exports.

This completes both codec directions for FSE in the migration crate and lets
the unchanged C compatibility tests exercise the Rust encoder implementation.

Test Plan:
- cargo clippy; cargo clippy --benches; cargo clippy --tests
- cargo +nightly fmt, then repeat the Clippy checks
- cargo test --all-targets
- cargo build --release
- Compile the C shim with strict warnings enabled
- 1,000-case pristine-C differential for tables, headers, and payloads
- ./tests/fuzzer -v -T10s
- ./tests/decodecorpus -t -T5

Refs: rust/README.md

rust/README.md

@@ -20,6 +20,8 @@ zstd ABI:
 - Entropy coding
   - `entropy_common` reads FSE normalized counts and Huffman statistics.
   - `fse_decompress` builds FSE decoding tables and decodes FSE streams.
+  - `fse_compress` normalizes counts, writes FSE headers, builds compression
+    tables, and encodes FSE streams.
 - Compression primitives
   - `hist` counts byte frequencies for FSE and Huffman compression.
 - Runtime support