feat(web): complete PBR shader, asset library and offscreen render chain

- generate a desktop-Blender PBR asset library and seed catalog v2 into OPFS
- add React asset browser, material preview sphere and Web PBR render panel
- extend shader graph/compiler to the finite 14-node set and native properties
- add image colour-space editing, scene-camera still rendering and asset repair
- add focused E2E/unit coverage and implementation status notes

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

web/protocol/ui-schema.ts

@@ -25,6 +25,7 @@ export type EditorType =
   | "Compositor"
   | "Geometry Nodes"
   | "File Browser"
+  | "Asset Library"
   | "Spreadsheet"
   | "Text Editor"
   | "Python Console"