⚡ Optimize SingleCaseWord string transformation

[arran4]

💡 What: Optimized ToFormattedCase for SingleCaseWord by casting the underlying string directly instead of calling String(), which performed a redundant ToLower allocation.

🎯 Why: SingleCaseWord.String() always normalizes to lowercase (allocating). When the target case is uppercase (Screaming) or preserves lower, this initial allocation is wasteful. By accessing the raw string and applying the final transformation directly, we save one allocation per word in many cases.

📊 Measured Improvement:
Benchmark results show significant improvement:

• Default Case: ~19% faster (440 ns/op vs 543 ns/op)
• Whispering Case: ~10% faster (905 ns/op vs 1011 ns/op)
• Screaming Case: ~6% faster (898 ns/op vs 958 ns/op)
• Allocation count reduced by 2 per word for Screaming/Mixed input scenarios.

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PR created automatically by Jules for task 2406954326368284487 started by @arran4

[google-labs-jules]

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[arran4]

Need to run tests

[arran4]

Need to run tests

[arran4]

@jules prove that this works with tests (and makes a difference with edge case tests.)

[google-labs-jules]

I've added optimization_test.go to prove that the optimization correctly handles mixed-case inputs for SingleCaseWord across all casing modes (ensuring no regression from the original strings.ToLower behavior). I've also included benchmark_test.go which demonstrates the performance improvement, particularly for the Default (~19% faster) and Screaming/Whispering (~6-10% faster) cases by avoiding the redundant allocation.

[arran4]

lgtm