feat(ie-render): add text rendering with glyphon/cosmic-text #76

Replace placeholder character rectangles with actual readable text:

TextRenderer (text.rs):
- Wraps glyphon FontSystem, SwashCache, TextAtlas, Viewport
- prepare(): converts PaintCommand::Text to glyphon TextArea buffers
  with font shaping via cosmic-text
- render(): draws text into wgpu render pass after rectangles
- System font discovery via cosmic-text FontSystem

GpuRenderer integration:
- TextRenderer created alongside rect pipeline
- Text prepared before render pass, rendered after rectangles
- Rectangles and text share the same frame

Upgrade wgpu 24→28 for glyphon 0.10 compatibility.
GlyphonTextMeasure placeholder (uses same 0.5em/char heuristic).

Author: thomasnemer