Step 6b: Text rendering with glyphon/cosmic-text

[thomasnemer]

Parent: #25
Prerequisites: Step 6a (#74)

Goal

Replace placeholder character rectangles with actual readable text using glyphon and cosmic-text for font shaping and glyph rasterization.

Current state

Text is rendered as small colored rectangles per character (software.rs render_text_simple). This validates the pipeline but is unreadable.

Implementation

• Add glyphon + cosmic-text dependencies
• TextRenderer struct wrapping glyphon components (FontSystem, SwashCache, TextAtlas)
• Implement ie_layout::TextMeasure trait using cosmic_text::FontSystem for accurate text measurement
• Replace render_text_simple with glyphon-based text rendering into the softbuffer pixel buffer
• System font discovery and loading

Acceptance Criteria

• Text is readable in the browser window
• Font sizes from CSS are respected
• Bold/italic rendering works
• TextMeasure provides accurate widths for layout