feat(paint-slots): per-part paint for windows + doors, chrome/brass, world-scale UVs

Builds on the explicit per-mesh slot tagging (currentDoorSlot/currentWindowSlot):

- Per-part painting: door = panel/frame/glass/hardware, window = frame/glass,
  each independently paintable. The recessed door/window body sits behind the
  wall, so the proud invisible cutout wins the scene raycast over the wall and
  the shared resolveSlotByReRaycast() re-raycasts the kind's own subtree to pick
  the exact part under the cursor (panel↔frame↔glass↔hardware). Hover tracks the
  cursor via a  re-eval (idempotent, no flicker).
- Door frame is its own slot (separate frameMaterial); hardware = new flat
  'metal-chrome'.
- Library defaults (generic): panel/frame -> library:preset-softwhite, glass ->
  library:preset-glass (flipped preset-glass to FrontSide — DoubleSide poisons
  the WebGPU MRT pass; it's the only glass we use).
- Catalog: add flat (non-PBR) 'metal-chrome' + 'metal-brass'; drop metal
  metalness 1 -> 0.6 so metals are lit by existing lights (no env needed).
- World-scale UVs (1 unit = 1m) on door/window box meshes via shared box-uv.ts,
  so finishes tile at real-world scale instead of stretching.
- PaintResolveArgs gains an optional  for subtree re-raycasting.

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

packages/viewer/src/lib/box-uv.ts

@@ -0,0 +1,40 @@
+import type { BoxGeometry } from 'three'
+
+/**
+ * Rewrite a default `BoxGeometry`'s UVs to world scale — 1 UV unit = 1 metre —
+ * so tiled finishes (with `repeat` in tiles-per-metre) render at a consistent
+ * real-world scale instead of stretching to fit each face. Matches the
+ * world-scale UV convention used by the procedural slab/wall geometry.
+ *
+ * three.js builds box faces in the fixed order [+X, -X, +Y, -Y, +Z, -Z], four
+ * verts each, with UVs spanning 0→1 across the face. Each face's two in-plane
+ * dimensions differ, so we scale U/V per face by that face's size in metres.
+ */
+export function applyWorldScaleBoxUVs(
+  geometry: BoxGeometry,
+  w: number,
+  h: number,
+  d: number,
+): void {
+  const uv = geometry.getAttribute('uv')
+  if (!uv || uv.count < 24) return // non-default segmentation — leave as-is
+
+  // [uScaleMetres, vScaleMetres] per face, in three's face order.
+  const faceScale: Array<[number, number]> = [
+    [d, h], // +X
+    [d, h], // -X
+    [w, d], // +Y
+    [w, d], // -Y
+    [w, h], // +Z
+    [w, h], // -Z
+  ]
+
+  for (let face = 0; face < 6; face += 1) {
+    const [us, vs] = faceScale[face]!
+    for (let v = 0; v < 4; v += 1) {
+      const i = face * 4 + v
+      uv.setXY(i, uv.getX(i) * us, uv.getY(i) * vs)
+    }
+  }
+  uv.needsUpdate = true
+}