Add per-constraint delete; fix named-parameter reference bugs

Constraint delete: a Properties-panel row delete button, viewport
click-to-select on a constraint glyph pill with Delete-key support, and a
right-click "Delete Constraint" menu item — all routed through one
undoable Scene::selected_constraint-aware path.

Fixes two named-parameter bugs found while testing that feature:
- DIMCONSTRAINT: a parameter named like a mode-switch keyword ("r", "d",
  "a", "x", "y") could never be referenced, since the keyword check ran
  before the parameter-name check.
- PARAMETERS modal Apply rebuilt Scene::named_parameters but never
  refreshed the Properties panel, leaving renamed/deleted rows stale
  until an unrelated refresh (e.g. a selection change) happened to occur.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -4287,6 +4287,13 @@ impl OpenCADStudio {
                 }
                 let i = self.active_tab;
                 self.tabs[i].scene.selection.borrow_mut().context_menu = None;
+                // A selected constraint-glyph pill takes Delete before entity
+                // erase — the two selections are mutually exclusive (see
+                // `Scene::selected_constraint`).
+                if let Some(id) = self.tabs[i].scene.selected_constraint {
+                    self.delete_sketch_constraint(id);
+                    return Task::none();
+                }
                 let handles: Vec<_> = self.tabs[i].scene.selected.iter().cloned().collect();
                 if !handles.is_empty() {
                     // Erase is delta-safe unless a target is in a group (group
@@ -6218,6 +6225,10 @@ impl OpenCADStudio {
                 self.refresh_properties();
                 Task::none()
             }
+            Message::PropConstraintDelete(id) => {
+                self.delete_sketch_constraint(id);
+                Task::none()
+            }
 
             // ── Options / About windows ───────────────────────────────────
             Message::OptionsOpen => {