⚡ Bolt: [performance improvement] eliminate intermediate Vec allocations in string casing

[cloudesize67-cmd]

💡 What:

• Replaced .collect::<Vec<_>>().join("") with .collect::<String>() in xdk-lib/src/casing.rs when generating PascalCase strings.
• Replaced .to_lowercase().collect::<String>() with .to_lowercase().to_string() in xdk-lib/src/casing.rs when lowercasing the first character of a string.

🎯 Why:

• Collecting an iterator of strings into an intermediate Vec only to immediately join them performs unnecessary heap allocations. Using collect::<String>() directly utilizes the FromIterator implementation for String which efficiently appends the iterator's contents.
• Using collect::<String>() on the ToLowercase iterator of a single char involves more overhead than just calling .to_string(), which leverages the Display implementation directly.

📊 Impact:

• Eliminates intermediate Vec allocations during string manipulation in SDK generation (specifically when generating PascalCase variables/classes), reducing memory churn and improving execution speed.
• Micro-optimization for single character lowercasing.

🔬 Measurement:

• Run make check and make test-generator to ensure all functionality remains identical. No regressions introduced.

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PR created automatically by Jules for task 1629076768070896470 started by @cloudesize67-cmd

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