Changes:

• Removed dependency on string.ascii_lowercase and string.ascii_uppercase and Replaced string-based capitalization with manual ASCII offset
• Reduced function complexity and branching
• Added basic unit tests to ensure correctness

Benefits:

• Eliminated external module dependency (string) for better portability and reduced overhead
• Improved performance (×ばつ faster in benchmarks)
• More efficient implementation
• Maintains readability and correctness

Checklist:

• Code compiles without errors
• Performance verified with benchmarks