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/scene/selection.rs

@@ -72,6 +72,7 @@ impl Scene {
         if handles.is_empty() {
             return;
         }
+        self.selected_constraint = None;
         let expanded = self.expanded_with_leaders(handles);
         let mut changed = false;
         for handle in expanded {
@@ -111,6 +112,7 @@ impl Scene {
         expanded
     }
     pub fn select_entity(&mut self, handle: Handle, exclusive: bool) {
+        self.selected_constraint = None;
         let handles = self.handles_expanded_for_leader_annotations(&[handle]);
         let mut changed = false;
 
@@ -133,6 +135,7 @@ impl Scene {
     }
 
     pub fn deselect_all(&mut self) {
+        self.selected_constraint = None;
         if self.selected.is_empty() {
             return;
         }
@@ -215,6 +218,7 @@ impl Scene {
     /// only when its contents actually changed. History/file/command paths must
     /// use this instead of assigning `selected` directly.
     pub(crate) fn replace_selection(&mut self, selected: HashSet<Handle>) {
+        self.selected_constraint = None;
         let handles: Vec<Handle> = selected.iter().copied().collect();
         let selected: HashSet<Handle> = self
             .handles_expanded_for_leader_annotations(&handles)