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/properties.rs

@@ -2224,7 +2224,7 @@ impl OpenCADStudio {
                                     crate::scene::model::object::Property {
                                         label,
                                         field: "sketch_constraint",
-                                        value: crate::scene::model::object::PropValue::EntityLink { handles, conflicting },
+                                        value: crate::scene::model::object::PropValue::EntityLink { id: c.id, handles, conflicting },
                                     }
                                 })
                                 .collect();