feat(ie-render): implement display list and softbuffer page painting #74

First visual milestone — actual web page content rendered on screen:

Display list (paint.rs):
- PaintCommand: FillRect and Text
- build_display_list walks layout tree, emits background colors,
  borders (as thin rects), and text commands
- Respects visibility: hidden

Software rasterizer (software.rs):
- render_to_buffer produces ARGB pixel buffer from paint commands
- fill_rect with alpha blending
- Placeholder text: character-shaped colored rectangles
- SoftwareTextMeasure matching layout engine metrics

Shell integration (app.rs):
- Full rendering pipeline on RedrawRequested:
  HTML parse → style resolution (UA + <style>) → layout → display
  list → software rasterize → softbuffer present
- Falls back to dark background when no tab content

Removed wgpu dependency from ie-render (will be added back for
GPU rendering later). Added ie-html/ie-css/ie-layout/ie-render
as ie-shell dependencies.

4 render tests + all existing tests pass.

Author: thomasnemer