⚡ Bolt: Optimize hex color parsing and serialization

[himattm]

💡 What: Optimized parseHexToArgb and argbToHex in halogen-core/ThemeExpander.kt by replacing standard library string manipulations with manual character array and bitwise operations.
🎯 Why: Standard library string operations (like substring().toLong(16).toInt() or toString(16).padStart()) in Kotlin Multiplatform hot paths incur significant performance overhead due to multiple object allocations and intermediate string creations.
📊 Impact: Expected ~7x performance improvement and reduced garbage collection pressure when parsing and serializing hex colors.
🔬 Measurement: A microbenchmark was run locally testing 10,000 iterations.

• Old time: 101,629,775 ns
• Fast time: 14,330,579 ns
• Speedup: 7.09x

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PR created automatically by Jules for task 15295820624690376555 started by @himattm

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