Initial implementation: classical cryptanalysis toolkit

- Frequency analysis (single char, bigram, trigram)
- Index of coincidence for cipher type detection
- Kasiski examination for key length estimation
- Caesar cipher encrypt/decrypt/break
- Vigenere cipher encrypt/decrypt/break
- XOR analysis with single-byte and repeating-key breaking
- Shannon entropy calculation
- Chi-squared statistical testing
- Hamming distance (bytes and strings)
- English plaintext scoring and detection
- 133 tests