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/app/commands/display.rs

@@ -2549,6 +2549,7 @@ mod tests {
             instance_transform: None,
             instance_handle: None,
             instance_color: None,
+            edge_color: None,
             instance_aabb: None,
         }
     }