feat: add Gronsfeld cipher implementation

[qsrbit]

Description

This PR adds a Gronsfeld Cipher implementation to the ciphers module.

About Gronsfeld Cipher

The Gronsfeld cipher, a Vigenère variant where the key is a digit sequence (0-9). Includes 20 tests covering key validation, case preservation, alphabet wrap-around, and round-trip correctness.

Implementation Details

The implementation provides two public functions:

1. gronsfeld_encrypt(text: &str, key: &str) -> Result<String, &'static str>

   • Encrypts plaintext by shifting each letter forward by the corresponding key digit
   • Validates the key: must be non-empty and contain only digits (0–9)
   • Preserves the case of alphabetic characters
   • Passes non-alphabetic characters (spaces, punctuation, Unicode) through unchanged
   • Cycles the key if it is shorter than the plaintext

2. gronsfeld_decrypt(text: &str, key: &str) -> Result<String, &'static str>

   • Decrypts ciphertext by shifting each letter backward by the corresponding key digit
   • Same key validation and passthrough rules as encryption
   • Inverse of gronsfeld_encrypt: decrypt(encrypt(text, key), key) == text

Algorithm Details

• Character Set: Works with all ASCII alphabetic characters (A–Z, a–z)
• Key Format: Digit string, e.g. "31415" or "9876543210"
• Key Cycling: The key repeats cyclically when shorter than the text; only alphabetic characters consume a key digit
• Shift Range: Each digit shifts by 0–9 positions (a strict subset of Vigenère's 0–25)
• Case Preservation: Uppercase input stays uppercase, lowercase stays lowercase
• Time Complexity: O(n) where n is the length of the input text
• Space Complexity: O(n) for storing the result

Type of change

• New cipher implementation
• Documentation (docstrings and examples)
• Tests

Checklist:

All tests pass successfully:

cargo test gronsfeld
cargo fmt --check
cargo clippy -- -D warnings

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines