Problem

`getMaterialForOriginal` in `ItemRenderer` returns `baseMaterial` (`#f2f0ed`, roughness 0.5) for every mesh regardless of its source material. The GLTF's original colors, textures, PBR properties, and per-mesh material variation are all thrown away — every loaded model renders as a uniform cream-coloured shape with no visual distinction between parts.

The intent was to convert regular `MeshStandardMaterial` (from GLTFLoader) to `MeshStandardNodeMaterial` (required by the WebGPU/TSL pipeline). The implementation just returned a shared constant instead of actually converting the material's properties.

Fix

Rewrites `getMaterialForOriginal` to build a real `MeshStandardNodeMaterial` from each original material, copying:

• `color`, `roughness`, `metalness`, `emissive`, `emissiveIntensity`
• `opacity`, `transparent`, `alphaTest`, `side`, `vertexColors`
• All texture maps: `map`, `normalMap`, `roughnessMap`, `metalnessMap`, `emissiveMap`, `aoMap`, `alphaMap`

Properties are assigned explicitly after construction rather than passed through the constructor — `Material.setValues` silently drops certain fields on NodeMaterial subclasses (observed: `color` being ignored, producing white output for GLTFs whose material relies solely on `baseColorFactor` like Kenney's low-poly kits).

Results are cached per original material via `WeakMap`. A double-conversion guard detects materials that have already been replaced with a NodeMaterial on a previous render and returns them as-is.

Scope

• One file: `packages/viewer/src/components/renderers/item/item-renderer.tsx`
• 44 insertions, 3 deletions — the old `getMaterialForOriginal` (2 lines) is replaced with a proper converter (~40 lines)
• No schema changes, no store changes, no new dependencies
• Glass special-case preserved: materials named `glass` still get the shared `glassMaterial`

How to test

1. `bun dev`, load any scene with `ItemNode`s that reference real GLTF models.
2. Items should render with their GLTF's actual colors and textures — wood grain on a cabinet, fabric on a chair, metal on a desk. On `main`, every item renders as cream `#f2f0ed`.
3. Items with glass materials should still render translucent.
4. `bun check`, `bun check-types`, `bun run build` all clean.

Checklist

• I've tested this locally with `bun dev`
• My code follows the existing code style (`bun check` passes on the touched file)
• I've updated relevant documentation (N/A — no docs affected)
• This PR targets the `main` branch