docs(architecture): add item-authoring guide for paintable GLBs

Documents the content-author contract for catalog item GLBs: slot_
material naming + deriveSlotId rules, authored defaults and
pascal_material extras, the cutout reserved mesh, the ~1 UV unit/m world
scale, vertex colours for fixed detail, and the validated Blender + glTF
export recipe (Custom Properties export gotcha, gltfpack -ke extras
preservation). Indexes the page in the architecture README and links it
from materials-and-themes.md (previously a dangling "phase-6 Blender
recipe" reference).

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

wiki/architecture/README.md

@@ -11,6 +11,7 @@ Canonical rules for code that touches `packages/core`, `packages/viewer`, `packa
 | [renderers](renderers.md) | Node renderer pattern in `packages/viewer` |
 | [node-definitions](node-definitions.md) | Three-checkbox composition model for registry-driven kinds (`geometry` / `renderer` / `system`) |
 | [materials-and-themes](materials-and-themes.md) | Surface colour: surface roles, colour presets, the textures axis, and scene themes (appearance / ground / clay tints) |
+| [item-authoring](item-authoring.md) | Content-author contract for catalog item GLBs: `slot_` material naming, authored defaults + `pascal_material` extras, the `cutout` reserved mesh, UV world scale, and the validated Blender/export recipe |
 | [plugin-authoring](plugin-authoring.md) | Public contract for external plugins — `Plugin` shape, `setPluginDiscovery`, lifecycle, what's in and out of v1 |
 | [tools](tools.md) | Editor tools structure, 2D↔3D behavioral parity, manipulation constraints, and Shift bypass defaults |
 | [viewer-isolation](viewer-isolation.md) | Keeping `@pascal-app/viewer` editor-agnostic |