fix(paint-slots): guard empty fence slot group before mergeGeometries

mergeGeometries throws on an empty array, so an absent slot group (infill with
showInfill off, base on a floating fence) crashed the geometry build. Return an
empty BufferGeometry for an empty group; the renderer already skips meshes with
no position attribute.

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

packages/viewer/src/systems/fence/fence-system.tsx

@@ -252,6 +252,10 @@ function createFenceParts(fence: FenceNode): FenceSlotParts {
 }
 
 function mergeFenceParts(parts: FencePart[]): THREE.BufferGeometry {
+  // An empty slot group (e.g. infill with showInfill off, or base on a floating
+  // fence) must not reach mergeGeometries — it throws on an empty array. The
+  // empty geometry has no position attribute, so the renderer skips its mesh.
+  if (parts.length === 0) return new THREE.BufferGeometry()
   const geometries = parts.map(createFencePartGeometry)
   const merged = mergeGeometries(geometries, false) ?? new THREE.BufferGeometry()
   geometries.forEach((geometry) => {