⚡ Bolt: [performance improvement] optimize string concatenation

[cloudesize67-cmd]

💡 What: Replaced .collect::<Vec<_>>().join("") with .collect::<String>() and .collect::<String>() with .to_string() in xdk-lib/src/casing.rs.

🎯 Why: To prevent unnecessary intermediate vector allocations during string concatenation and character conversion, lowering memory overhead and slightly speeding up casing operations.

📊 Impact: Reduces memory overhead directly by removing intermediate vector allocations in highly utilized casing code paths.

🔬 Measurement: Verify by running make check and make test-generator. No regressions in functionality.

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PR created automatically by Jules for task 12886628493641816118 started by @cloudesize67-cmd

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