feat(paint-slots): world-scale pool slab floor UVs + document the metre UV contract

Pool slabs (generatePoolGeometry) were the one procedural surface emitting
normalized [0..1] floor UVs, so a textured finish stretched to fit the pool
instead of tiling at real-world scale like every other surface. Switch the
floor to shape-space metres (x, -z) — the same mapping generatePositiveSlab-
Geometry already uses for its caps. Pool walls were already in metres.

Also document the contract in wiki/architecture/materials-and-themes.md: every
procedural surface generates UVs in metres (1 UV unit = 1 m), GLB slots follow
the same ~1 unit/m authoring convention, and a catalog material's `repeat` is
therefore a per-material world-scale setting (tiles per metre), identical for
every surface that uses it — never per-item.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/viewer/src/systems/slab/slab-system.tsx

@@ -254,23 +254,12 @@ function generatePoolGeometry(slabNode: SlabNode): THREE.BufferGeometry {
   const positions: number[] = []
   const uvs: number[] = []
   const indices: number[] = []
-  const bounds = new THREE.Box2()
-
-  for (const [x, z] of polygon) {
-    bounds.expandByPoint(new THREE.Vector2(x, z))
-  }
-  for (const hole of holePolygons) {
-    for (const [x, z] of hole) {
-      bounds.expandByPoint(new THREE.Vector2(x, z))
-    }
-  }
-
-  const floorWidth = Math.max(bounds.max.x - bounds.min.x, 0.001)
-  const floorHeight = Math.max(bounds.max.y - bounds.min.y, 0.001)
 
   const pushFloorVertex = (x: number, y: number, z: number) => {
     positions.push(x, y, z)
-    uvs.push((x - bounds.min.x) / floorWidth, (z - bounds.min.y) / floorHeight)
+    // Floor UVs in metres (shape-space x, -z), matching generatePositiveSlabGeometry's
+    // cap mapping so a finish tiles at the same world scale on every surface.
+    uvs.push(x, -z)
   }
 
   const pushWallVertex = (x: number, y: number, z: number, u: number, v: number) => {