fix(viewer): harden WebGPU slab recovery

[Aymericr]

Summary

• copy slab primary UVs into a dedicated uv2 attribute after face splitting
• reset the renderer target when an MRT post-processing pass throws
• add a regression test covering uv2 on both slab meshes

This ports the two still-relevant fixes from #464 onto current main and credits Connor Soohoo in the commit. The empty-roof part of #464 is already superseded by the broader export hardening in #466.

Verification

• bun test src/slab/tests/geometry.test.ts src/slab/tests/definition.test.ts (from packages/nodes)
• bun run check
• bun run check-types
• bun run build

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Note

Medium Risk
Touches per-frame WebGPU render error handling and slab GPU geometry attributes; limited scope but affects viewer stability when MRT passes fail.

Overview
Hardens WebGPU rendering for slabs and post-processing recovery paths.

When slab geometry is split into top/side meshes, splitSlabFacesByFacing now duplicates primary uv into uv2 on both sub-geometries (same pattern as columns/roofs), so node-material / MRT pipelines don’t hit missing uv2 bindings. A regression test asserts uv2 matches uv on both slab meshes from buildSlabGeometry.

If the WebGPU MRT post-processing render() throws, the viewer calls setRenderTarget(null) before disposing the pipeline and retrying/falling back, so a stuck MRT target doesn’t poison the direct renderer.render fallback.

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