feat(statusbar): isolate / hide objects with status pill

Add a hidden-entity set to the scene that the tessellation visibility
test skips, so hidden objects neither render nor hit-test. New commands
ISOLATEOBJECTS (show only the selection), HIDEOBJECTS (hide the
selection) and UNISOLATEOBJECTS (restore). An ISO status pill lights up
while anything is hidden and restores everything on click.

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

src/app/view.rs

@@ -1159,6 +1159,7 @@ impl OpenCADStudio {
                         self.clean_screen,
                         tab.scene.document.header.insertion_units,
                         self.units_popup_open,
+                        tab.scene.is_isolation_active(),
                         &self.statusbar_config,
                     )
                 })