feat(paint-slots): KTX2 finish library — fabric/leather/concrete/metal (phase 4)

Wire KTX2 into the catalog finish path and add 15 textured finishes across four
new families, all as GPU-compressed KTX2 (512px) + a webp picker thumbnail.

- Shared `ktx2-loader.ts`: one KTX2Loader (Basis transcoder) used by both the
  GLB loader and catalog textures; `ensureKtx2Support(renderer)` runs once at
  viewer init (GPUDeviceWatcher) so `.ktx2` finishes load even with no GLB in
  the scene. `use-gltf-ktx2` now reuses the shared instance.
- `materials.ts`: texture loaders pick the KTX2 loader for `.ktx2` urls, the
  image loader otherwise (all three load paths).
- `material-library.ts`: 15 entries (fabric ×6, leather ×2, concrete ×4,
  metal ×3). Neutral albedo for tinting, `flipY: false` (compressed textures
  can't flip), `repeat` per real-world tile size, metals `metalness: 1`.
  Normals encoded UASTC, data maps ETC1S/linear.

Assets generated from raw sources via the new community
`scripts/build-material-textures.ts`. 1024/256 tiers + raws kept out of git.

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

packages/viewer/src/components/viewer/index.tsx

@@ -13,6 +13,7 @@ import * as THREE from 'three/webgpu'
 import { hasDrawableGeometry } from '../../lib/drawable-geometry'
 import { PERF_OVERLAY_ENABLED, pushGpuSample } from '../../lib/gpu-perf'
 import { applyIsolation, clearIsolation } from '../../lib/isolation'
+import { ensureKtx2Support } from '../../lib/ktx2-loader'
 import type { ColorPreset, RenderShading } from '../../lib/materials'
 import { getSceneTheme } from '../../lib/scene-themes'
 import useViewer, { type RenderContext } from '../../store/use-viewer'
@@ -144,6 +145,11 @@ function GPUDeviceWatcher() {
   const gl = useThree((s) => s.gl)
 
   useEffect(() => {
+    // Detect KTX2 transcode support as soon as the renderer exists, so catalog
+    // `.ktx2` finish textures load even in scenes with no GLB items (whose
+    // loader would otherwise be the only thing to call this).
+    ensureKtx2Support(gl)
+
     const backend = (gl as any).backend
     const device = backend?.device as WebGPUDeviceLike | undefined