Fix room surfaces, wall openings, and paint scope (#498)

* chore(core): point material catalog at KTX2 tiers for wood/flooring/roofing finishes

All 48 remaining webp/jpg/png finish entries now reference _512.ktx2 maps
and 256px _thumb.webp previews, matching the fabric/leather/concrete/metal
convention. flipY set to false on the converted entries — compressed
textures can't be flipped at upload.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix: stabilize room surfaces and wall openings

* style(core): format KTX2 material catalog

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/nodes/src/slab/definition.ts

@@ -18,7 +18,7 @@ import { SlabNode } from './schema'
 import { slabSlots } from './slots'
 
 const HEIGHT_HANDLE_OFFSET = 0.22
-const MIN_SLAB_ELEVATION = 0.02
+const MIN_SLAB_ELEVATION = -1
 
 function polygonVertexAverage(polygon: SlabNodeType['polygon']): [number, number] {
   if (polygon.length === 0) return [0, 0]
@@ -89,10 +89,10 @@ function slabHandleAnchor(slab: SlabNodeType): [number, number] {
 }
 
 // Slab height arrow — vertical chevron on solid slab surface near the
-// polygon center. Drags elevation (the extrusion thickness) with
-// `anchor: 'min'` so the bottom stays at world Y=0 and the top follows
-// the pointer. Same registry-handle pipeline as the column height arrow,
-// so live override + commit-on-release come for free.
+// polygon center. Drags elevation through zero: positive values extrude
+// upward from ground while negative values create a recessed floor whose
+// depth follows the pointer. Same registry-handle pipeline as the column
+// height arrow, so live override + commit-on-release come for free.
 function slabHeightHandle(): HandleDescriptor<SlabNodeType> {
   return {
     kind: 'linear-resize',