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/viewport.rs

@@ -4451,6 +4451,31 @@ properties={:.1}ms picked={}",
                 sel.box_current = None;
             } else {
                 if box_anchor.is_none() {
+                    // A sketch-constraint glyph pill draws on top of the
+                    // geometry it touches and takes priority over it when
+                    // clicked (design mirrors `has_selection` gating the
+                    // viewport right-click menu, and `DeleteSelected`
+                    // preferring a selected pill over entity erase).
+                    let glyph_hit = (self.tabs[i].scene.current_layout == "Model")
+                        .then(|| {
+                            let scope = self.tabs[i].current_sketch_scope();
+                            self.tabs[i].scene.constraint_glyph_hit(
+                                scope,
+                                canvas_sz,
+                                self.show_constraint_values,
+                                p_full,
+                            )
+                        })
+                        .flatten();
+                    if let Some(id) = glyph_hit {
+                        if !selection_pick_add && !self.shift_down {
+                            self.tabs[i].scene.deselect_all();
+                        }
+                        self.tabs[i].scene.selected_constraint = Some(id);
+                        self.refresh_properties();
+                        selection_just_completed = true;
+                    }
+                    if glyph_hit.is_none() {
                     let (view_rot, eye, all_wires) = self.pick_view(i, &edit_cam, bounds);
                     let click_world = self.cursor_model_point(i, &edit_cam, p, bounds);
                     let t_arm = crate::perf::enabled().then(Instant::now);
@@ -4611,6 +4636,7 @@ was_selected={}",
                             }
                         }
                     }
+                    }
                 } else {
                     let a = box_anchor.unwrap();
                     let box_points = [