* fix(huffman): Handle edge cases and improve error handling
- Change HuffmanDictionary::new() to return Option<Self> for safer API
- Add proper handling for empty alphabet (returns None)
- Add special case handling for single-symbol alphabets
- Replace unwrap() calls with ? operator in decode() for better error handling
- Add #[inline(always)] optimization for frequently called get_bit()
- Add comprehensive tests for edge cases
- Improve documentation with usage examples
BREAKING CHANGE: HuffmanDictionary::new() now returns Option<Self>
* Test: Increase coverage for huffman_encoding.rs decode method
Adds two new test cases to ensure 100% patch coverage for HuffmanEncoding::decode:
1. test_decode_empty_encoding_struct: Covers the edge case where num_bits == 0.
2. minimal_decode_end_check: Ensures the final 'if self.num_bits > 0' check in the multi-symbol decode path is fully covered.
Corrects 'char-lit-as-u8' and 'unnecessary-cast' lints in the newly added coverage tests to satisfy GitHub Actions.