fix(thickness): edit persists + wide polyline extrudes as a solid tube

Entering a thickness (DXF 39 extrusion) on a wide polyline now works end
to end:

- The properties Thickness field is a General-group common prop, so it is
  routed to apply_common_prop (set_entity_thickness) instead of the
  geometry setter, which ignored it — the value was silently dropped and
  reverted to 0.
- A wide polyline extrudes its whole band into a 3-D tube (outer + inner
  walls with top and bottom caps), not just its centreline.
- The tube is real 3-D geometry, not a flat draw-order hatch: it renders
  solid in every view mode with true depth, so nothing shows through the
  walls. The flat 2-D band fill is skipped for a thickened polyline.
- A fill whose triangles span depth keeps its real depth (no draw-order
  bias), so the extrusion's own edge wires stay in front of the surface
  and remain visible instead of only the silhouette.
- The corner and cap edge lines read black, framing the coloured tube.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/face3d_gpu.rs

@@ -211,7 +211,18 @@ impl Face3DGpu {
                 }
             } else {
                 let fill_color = [r, g, b, a];
-                let depth = depth_of(wire);
+                // A fill whose triangles span depth is a genuine 3-D surface (an
+                // extruded polyline tube), not a flat coplanar overlay (2-D SOLID,
+                // greek text, dimension background). Keep its real depth so the
+                // fill occludes correctly and its own edge wires — coincident with
+                // the surface — win the depth test and stay visible. A flat overlay
+                // keeps the draw-order bias that layers it in screen order.
+                let (mut zmin, mut zmax) = (f32::INFINITY, f32::NEG_INFINITY);
+                for p in &wire.fill_tris {
+                    zmin = zmin.min(p[2]);
+                    zmax = zmax.max(p[2]);
+                }
+                let depth = if zmax - zmin > 1e-4 { 0.0 } else { depth_of(wire) };
                 for (i, &position) in wire.fill_tris.iter().enumerate() {
                     verts_2d.push(Face3DVertex {
                         position,