Add core Huffman test coverage

[nexiumbiz-debug]

Summary

• Add broader tests around core Huffman encode/decode behavior.
• Cover empty input, repeated-symbol input, punctuation/whitespace text, frequency helpers, relative frequencies, entropy, tree root weights, unique generated codes, and prefix-free codes.

Testing

• npm test (11 tests passed)
• node -e "const core=require('./packages/core/dist'); const text='hello, huffman!'; const codes=core.getCharCodesFromSource(text); const encoded=core.encode(text,codes); const decoded=core.decode(encoded,codes); if(decoded!==text) throw new Error('round trip failed'); console.log('manual core round-trip ok');"

Closes #1