feat(rust): port FSE entropy decoding

Move FSE normalized-count parsing, Huffman statistics decoding, FSE table
construction, and FSE stream decompression into Rust. The C source files now
only retain the headers needed by the existing build configuration.

The implementation keeps the public C ABI and verifies C-generated balanced,
skewed, and Huffman-statistics streams. Compression and the higher-level frame
decoder remain C for now.

Test Plan:
- cargo fmt --check
- cargo test --all-targets
- cargo clippy --all-targets -- -D warnings
- cargo build --release
- compile both C shims with -Werror and -Wredundant-decls

Refs: rust/README.md

rust/README.md

@@ -17,10 +17,14 @@ zstd ABI:
   - `errors`, `debug`, `xxhash`, and `zstd_common` provide common exported ABI
     functions and state.
   - `common` contains shared frame constants and internal data types.
-All entropy coding, compression, decompression, dictionary, legacy, runtime,
-and CLI translation units are still C at this initial stage. They must move
-before the rewrite is complete. Keeping that boundary explicit prevents a
-passing hybrid build from being mistaken for the final all-Rust result.
+- Entropy coding
+  - `entropy_common` reads FSE normalized counts and Huffman statistics.
+  - `fse_decompress` builds FSE decoding tables and decodes FSE streams.
+
+The remaining compression, general decompression, dictionary, legacy, runtime,
+and CLI translation units are still C. They must move before the rewrite is
+complete. Keeping that boundary explicit prevents a passing hybrid build from
+being mistaken for the final all-Rust result.
 
 ## Compatibility boundary