feat(rust): port histogram counting primitives

Move byte histogram counting into Rust for FSE and Huffman compression callers.
The implementation preserves the C workspace contract, checked alphabet path,
fast path, empty-input behavior, and C ABI while the original C source becomes
a header-only compatibility shim.

Focused tests cover count accumulation, invalid alphabets, workspace failures,
and striped large inputs. Higher-level compression remains C for now.

Test Plan:
- cargo fmt --check
- cargo test --all-targets
- cargo clippy --all-targets -- -D warnings
- cargo build --release
- make -C tests fuzzer
- ./tests/fuzzer -i1 --no-big-tests
- compile hist.c with -Werror and -Wredundant-decls

Refs: rust/README.md

rust/README.md

@@ -20,14 +20,16 @@ zstd ABI:
 - Entropy coding
   - `entropy_common` reads FSE normalized counts and Huffman statistics.
   - `fse_decompress` builds FSE decoding tables and decodes FSE streams.
+- Compression primitives
+  - `hist` counts byte frequencies for FSE and Huffman compression.
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.
 
-The remaining compression, general decompression, dictionary, legacy, 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.
+The remaining block compression, general decompression, dictionary, legacy,
+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
 
@@ -56,8 +58,8 @@ Then run original compatibility tests from the repository root, starting with
 the narrow target for the component being migrated. For example:
 
 ```sh
-make -C tests poolTests
-./tests/poolTests
+make -C tests fuzzer
+./tests/fuzzer -i1 --no-big-tests
 ```
 
 Broader `tests/Makefile` targets remain the authoritative integration gates as