feat(render): solid fill for PolyfaceMesh and PolygonMesh entities

PolyfaceMesh and PolygonMesh were wireframe-only. They now render with
the same flat-color solid fill as Face3D entities.

- Add fill_tris: Vec<[f32;3]> to TruckEntity and WireModel
- PolyfaceMesh: fan-triangulate each face into fill_tris
- PolygonMesh: triangulate each grid quad into fill_tris
- tessellate.rs Lines branch: apply world_offset to fill_tris
- Face3DGpu::from_wires: collect fill_tris from all wires in addition
  to Face3D key_vertices quads
- upload_face3d: pass all_wires so fill_tris are batched into one draw call

src/entities/attribute.rs

@@ -42,6 +42,7 @@ impl TruckConvertible for AttributeDefinition {
             snap_pts: vec![(snap_pt, SnapHint::Insertion)],
             tangent_geoms: vec![],
             key_vertices: vec![],
+            fill_tris: vec![],
         })
     }
 }
@@ -146,6 +147,7 @@ impl TruckConvertible for AttributeEntity {
             snap_pts: vec![(snap_pt, SnapHint::Insertion)],
             tangent_geoms: vec![],
             key_vertices: vec![],
+            fill_tris: vec![],
         })
     }
 }