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/view/overlay.rs

@@ -1114,6 +1114,7 @@ pub(super) fn viewport_context_menu_overlay(
     bottom_inset: f32,
     has_cmd: bool,
     has_selection: bool,
+    selected_constraint: Option<crate::scene::sketch_constraints::ConstraintId>,
     isolation_active: bool,
     last_cmds: Vec<String>,
     draworder_open: bool,
@@ -1250,6 +1251,15 @@ pub(super) fn viewport_context_menu_overlay(
                 t!("Invert Selection").into_owned(),
                 Message::InvertSelection,
             ));
+        } else if let Some(id) = selected_constraint {
+            // A selected constraint-glyph pill gets its own minimal menu —
+            // none of the entity-selection actions above (Move/Copy/Isolate/
+            // Select Similar) apply to it.
+            items.push(item(
+                t!("Delete Constraint").into_owned(),
+                Message::PropConstraintDelete(id),
+            ));
+            items.push(sep());
         }
         if isolation_active {
             items.push(item(