feat: add Trifid cipher implementation

[AliAlimohammadi]

Description

This PR adds an implementation of the Trifid cipher to the ciphers module.

The Trifid cipher is a classical cipher that uses a table to fractionate each plaintext letter into a trigram, mixes the constituents of the trigrams, and then applies the table in reverse to turn these mixed trigrams into ciphertext letters.

Changes

• Added src/ciphers/trifid.rs with encryption and decryption functions
• Updated src/ciphers/mod.rs to include the new module and exports
• Includes comprehensive tests with 13 test cases covering:
  • Basic encryption and decryption
  • Custom alphabets
  • Round-trip encryption/decryption
  • Error handling for invalid inputs (invalid alphabet length, invalid characters)
  • Edge cases (empty messages, single characters, unmapped characters, empty input)
  • Internal helper functions with full code coverage

Features

• trifid_encrypt(message, alphabet, period) - Encrypts a message using the Trifid cipher
• trifid_decrypt(message, alphabet, period) - Decrypts a Trifid cipher encrypted message
• Proper error handling with Result<String, String>
• Configurable alphabet (must be 27 characters)
• Configurable period for grouping characters
• Type aliases for improved code clarity and reduced complexity

Testing

All tests pass with 100% code coverage:

cargo test trifid

References

• Wikipedia: Trifid Cipher

Checklist

• Code follows the repository's style guidelines
• All tests pass
• Added comprehensive test coverage (13 tests, 100% coverage)
• Updated mod.rs with new module
• Added documentation comments
• No clippy warnings
• Code formatted with cargo fmt

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.04%. Comparing base (fb5784f) to head (36a00cf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
+ Coverage   96.02%   96.04%   +0.02%     
==========================================
  Files         377      378       +1     
  Lines       27217    27392     +175     
==========================================
+ Hits        26135    26310     +175     
  Misses       1082     1082              

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[AliAlimohammadi]

@siriak, this is ready to be merged.