* Add catalog-based material presets across structural nodes
* Fix material picker fallback and wall visible highlights
* Preserve wall materials in selection highlights
- Add MaterialSchema with 10 presets (white, brick, concrete, wood, glass, metal, plaster, tile, marble, custom)
- Add material field to Wall, Slab, Door, Window, Ceiling, Roof, RoofSegment nodes
- Create MaterialPicker UI component with preset selection and custom properties
- Update all renderers to support material rendering with caching
- Add Material section to all node panels (WallPanel, SlabPanel, DoorPanel, WindowPanel, CeilingPanel, RoofPanel, RoofSegmentPanel)
- Update AGENTS.md with Material System documentation
