fix(viewport): adapt entity colours to visible bg

`model_wires_for_viewport` was tessellating with `self.bg_color` (the
model bg), so model content rendered inside a paper-layout viewport
adapted to a dark bg even though the transparent shader now reveals
the PaperCanvas sheet behind it — white wires stayed white on a light
sheet. Pass the paper bg in a paper layout.

Wire cache also needed an explicit invalidation when the bg or the
active layout changes; otherwise the cached, already-adapted wires
were returned against the new bg unchanged.

- BACKGROUND command bumps `geometry_epoch` after both branches
- new `Scene::set_current_layout` setter bumps on actual change; layout
  switch + history restore use it

src/app/commands.rs

@@ -51,6 +51,11 @@ impl OpenCADStudio {
                         self.tabs[i].bg_color = None;
                         self.tabs[i].scene.bg_color = [0.11, 0.11, 0.11, 1.0];
                     }
+                    // Wire colour adaptation (`adapt_to_bg`) reads the bg
+                    // at tessellation time, so the cached wires need to
+                    // refresh — otherwise a light→dark bg flip leaves
+                    // black lines invisible against the new bg.
+                    self.tabs[i].scene.bump_geometry();
                     self.command_line
                         .push_output("Background reset to default.");
                 } else if args.len() >= 3 {
@@ -64,6 +69,7 @@ impl OpenCADStudio {
                         self.tabs[i].bg_color = Some([r, g, b, 1.0]);
                         self.tabs[i].scene.bg_color = [r, g, b, 1.0];
                     }
+                    self.tabs[i].scene.bump_geometry();
                     self.command_line.push_output(&format!(
                         "Background: rgb({}, {}, {})",
                         args[0], args[1], args[2]