Implements the Kölner Phonetik algorithm for German word sound-encoding.
This phonetic algorithm is used to find gazetteer entries that sound
similar to a potentially misspelled token, enabling correction
suggestions.
The implementation includes:
- `encode`: The main function that orchestrates the encoding process.
- `prepare`: Prepares the input string by expanding umlauts,
uppercasing, and filtering non-alphabetic characters.
- `raw_encode`: Performs the character-by-character phonetic encoding
with context-aware rules.
- `postprocess`: Cleans up the raw output by collapsing duplicate digits
and removing leading zeros.
Includes unit tests to verify the correctness of the encoding for
various German words and edge cases, ensuring phonetic similarity and
handling of special character combinations.
