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/mod.rs

@@ -2908,6 +2908,12 @@ pub enum Message {
     /// A Constraints-section row was clicked: select every entity in the
     /// list (replacing the current selection) in the viewport.
     PropConstraintLinkClick(Vec<acadrust::Handle>),
+    /// A Constraints-section row's delete button was clicked, the Delete key
+    /// was pressed with the constraint's viewport glyph selected, or its
+    /// right-click "Delete Constraint" menu item was chosen: remove that one
+    /// persistent sketch constraint (undoable), independent of the entities
+    /// it touches.
+    PropConstraintDelete(crate::scene::sketch_constraints::ConstraintId),
     // ── About window ────────────────────────────────────────────────────
     AboutOpen,
     // ── Graphics warning ────────────────────────────────────────────────