fix(count): redraw only app-edited table blocks; recolour count mode in place

- refresh_table_block no longer runs for every table change: only tables
  whose picture reference this app dropped while editing them (and new
  tables without one), never on appearance-only bumps. Moved, restyled or
  freshly opened tables keep their authored *T block. TABLE edits now drop
  the picture reference like the other table edit paths.
- set_count_display recolours just the handles whose count class changed
  (restyle path + recolor_meshes_for_handles); only entering or leaving
  count mode rebuilds the whole drawing.
- Count colouring keeps insert instancing: counted and duplicate wires get
  their own instance groups instead of dropping render_instance.
- The hatch backdrop takes the count colour too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/entity.rs

@@ -1452,6 +1452,9 @@ impl Scene {
                     }
                 }
                 m.color = self.count_color(handle, m.color, self.current_bg());
+                if let Some(b) = &mut backdrop {
+                    b.color = self.count_color(handle, b.color, self.current_bg());
+                }
                 if tint_selected && self.selected.contains(&handle) {
                     m.color = [0.15, 0.55, 1.00, m.color[3]];
                 }