* Add catalog-based material presets across structural nodes
* Fix material picker fallback and wall visible highlights
* Preserve wall materials in selection highlights
The ceiling renderer was falling back to DEFAULT_CEILING_MATERIAL (an
opaque MeshStandardMaterial) when no material was set on the node.
This made ceilings fully opaque instead of using the intended
transparent grid pattern for scenes created before v0.3.2.
Now uses resolveMaterial() to extract the color from whatever material
config exists (preset, custom, or default white), then always creates
the proper transparent ceiling materials for both faces.
- 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
