fix(render): colour-7 solid edges contrast with a layout's paper

A solid's edges and silhouettes were drawn in its face colour, and the
edge shader read that from the material instead of the edge vertices.
Colour 7 is white, so in Wireframe 2D a solid vanished white-on-white in
a layout viewport (and on a light model background), while wires and
block solids already turned black there. Solids now keep a separate
edge colour, colour 7 turned against the background they are drawn on,
and the edge shader uses the edge vertices' colour again, which also
brings back visual-style edge colours. Shaded faces keep their colour
so they still read as lit. (#1446)

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/shaders/mesh.wgsl

@@ -594,7 +594,10 @@ fn fs_main(in: VertexOut) -> @location(0) vec4<f32> {
 // wireframe and hidden-line edge passes so lines read at their true colour.
 @fragment
 fn fs_edge(in: EdgeVertexOut) -> @location(0) vec4<f32> {
-    return vec4<f32>(surface.face_color.rgb, 1.0);
+    // The edge buffers carry their own colour (keyed by it, so instances
+    // share a buffer only with the same colour): a visual style's edge
+    // colour, or colour 7 turned against the background (#1446).
+    return vec4<f32>(in.color.rgb, 1.0);
 }
 
 // Edge fragment for filled render modes: force black so edges frame the shaded