Minimize text rendering allocations

[treeform]

Summary

• Decode text as UTF-8 directly instead of materializing rune strings.
• Add runtime Rune glyph lookup and packed kerning caches rebuilt from atlas data.
• Use the allocation-free lookup path in drawText and getTextSize.

Profile

Basic Window, 300 frames:

• Before: text rendering was about 15.4k allocations per steady frame.
• After: drawText and getTextSize are 0 allocations per steady frame.
• Frame steady average dropped from about 0.326 ms to about 0.084 ms.