⚡ Bolt: Optimize hex color parsing and formatting for performance

[himattm]

💡 What:
Optimized parseHexToArgb and argbToHex in halogen-core/src/commonMain/kotlin/halogen/ThemeExpander.kt to use explicit manual character array parsing and building instead of standard library string manipulation functions (substring, toLong(16), toString(16), padStart, uppercase).

🎯 Why:
To avoid the creation and garbage collection of multiple intermediate String objects during hex color conversion. This conversion happens frequently in the hot path of the theme generation process, making it a critical area to minimize allocation overhead.

📊 Impact:
Reduces allocations significantly per color processed, which cumulatively improves memory efficiency and raw performance when generating themes.

🔬 Measurement:
Standard benchmark or profiling of theme generation will show fewer allocations corresponding to these functions and faster execution times. Verified correctness by ensuring the core unit tests continue to pass (./gradlew :halogen-core:test).

---

PR created automatically by Jules for task 9052636535418590400 started by @himattm

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.