perf(anagram): optimize signature generation using character frequency

[sowndappan5]

Replaced sorting-based signature computation (O(m log m)) with a fixed-size character frequency tuple (O(m)) for anagram detection.

• signature() now generates a 26-length tuple representing letter counts
• dictionary keys changed from sorted strings to frequency tuples
• improves performance, especially for long words
• space complexity remains O(n*m), but avoids creating intermediate sorted strings
• ensures same functionality while reducing time complexity from O(nm log m) to O(nm)
• added inline comments for better readability

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