⚡ Bolt: Optimize hex color parsing and formatting

[himattm]

💡 What: Replaced standard library string methods (substring, toLong(16), toString(16), padStart, uppercase) with manual character array traversal and bitwise math in ThemeExpander.kt's hex color conversion functions. Added explanatory comments and logged a journal entry.

🎯 Why: Hex string manipulation happens frequently in theme generation hot paths. String slicing and regexes create unnecessary intermediate String allocations and invoke state machines, creating a measurable performance overhead.

📊 Impact: 4-10x performance improvement in parseHexToArgb and argbToHex (tested in scratchpad: Parse Old: 711ms -> Parse New: 65ms, Format Old: 330ms -> Format New: 74ms for 500k iterations). Reduces garbage collection pressure by eliminating intermediate object allocations.

🔬 Measurement: Verifiable via JMH or simple measureTimeMillis benchmarks targeting the modified parseHexToArgb and argbToHex functions. The change passes all existing unit tests, confirming accurate mathematical replication of the original behavior.

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PR created automatically by Jules for task 7986912674687494809 started by @himattm

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