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
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