⚡ Bolt: [performance improvement] Optimize string allocations in casing module

[cloudesize67-cmd]

💡 What: Replaced .collect::<Vec<_>>().join("") with .collect::<String>() and .to_lowercase().collect::<String>() with .to_lowercase().to_string() in xdk-lib/src/casing.rs.
🎯 Why: Avoids unnecessary intermediate Vec allocations and iterator collection overhead during string manipulation.
📊 Impact: Improves execution speed and reduces memory allocations for string casing operations.
🔬 Measurement: Verified via make check, cargo test -p xdk-lib, and make test-generator.

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PR created automatically by Jules for task 13683491660564881157 started by @cloudesize67-cmd

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