feat(materials): dynamic library registry, picker source tabs, Other category (#525)

* feat(materials): dynamic library registry, picker source tabs, create-material entry point

Core gains a runtime material registry (registerLibraryMaterials/
unregisterLibraryMaterials/subscribeLibraryMaterials) so embedders can
feed user/community materials; library: refs to registered materials
resolve in the viewer unchanged. MaterialCatalogItem carries an optional
source (pascal|community|mine|workspace). MaterialPicker gets a source
filter row and an optional onCreateMaterialRequest '+ New material'
tile, threaded through MaterialPaintPanel.

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

* fix(materials): underline source tabs in picker, matching catalog browse surfaces

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

* feat(materials): add 'other' category for uncategorized library materials

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

* fix(viewer): rebuild node material when a texture slot is cleared

Reused cached WebGPU materials keep a compiled TextureNode per slot; nulling
the slot for a cold texture load (or a preset without the map) without
needsUpdate leaves the node's per-frame material reference pulling null,
crashing the render pass in TextureNode.update. Cold loads are the norm for
freshly generated library materials, whose maps aren't in the texture cache.

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

* fix(viewer): standard materials always carry a displacement texture

three's WebGPU shadow pass copies each object's displacementMap onto one
shared per-light shadow material while caching per-mesh shadow node graphs
against that shared override. A mesh whose shadow graph was built with a
displacement TextureNode (painted with a generated material — the only
presets carrying height maps) crashes the render pass with "null (reading
'matrix')" the moment a material without a displacement texture is swapped
onto it, which is exactly what the paint hover preview does. Give every
MeshStandardNodeMaterial a shared 1x1 black displacement texel (zero offset)
when it has no real height map, so the copied slot is never null in either
direction.

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

---------

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

packages/core/src/index.ts

@@ -157,7 +157,9 @@ export {
 } from './lib/zone-quantities'
 export {
   getCatalogMaterialById,
+  getDynamicLibraryMaterials,
   getLibraryMaterialIdFromRef,
+  getLibraryMaterialsVersion,
   getMaterialPresetByRef,
   getMaterialsForCategory,
   getSceneMaterialIdFromRef,
@@ -168,12 +170,16 @@ export {
   type MaterialCatalogItem,
   type MaterialCategory,
   type MaterialRef,
+  type MaterialSource,
   type MaterialSurface,
   type ParsedMaterialRef,
   parseMaterialRef,
+  registerLibraryMaterials,
   SCENE_MATERIAL_REF_PREFIX,
+  subscribeLibraryMaterials,
   toLibraryMaterialRef,
   toSceneMaterialRef,
+  unregisterLibraryMaterials,
 } from './material-library'
 export type {
   FloorPlacedFootprint,