perf(engine): gate frame-hot ECS systems on resource changes

- find_draggable_at: break instead of return None on non-top non-tableau
  hit so remaining pile searches are not abandoned early (M-9)
- update_stock_count_badge: run only when GameStateResource changes (M-5)
- update_drop_highlights: run only when DragState changes (M-6)
- update_high_contrast_borders/backgrounds: run only when SettingsResource
  changes (M-7)
- update_selection_hud: run only when SelectionState or GameStateResource
  changes; uses resource_exists_and_changed to avoid panic in tests where
  SelectionState is not registered (M-8)
- Volume toast threshold: f32::EPSILON → 0.001 to avoid spurious toasts
  from float rounding noise in settings events (M-10)
- check_no_moves: collapse read().next().is_some() + clear() into a single
  read().count() > 0 drain (M-11)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/cursor_plugin.rs

@@ -80,7 +80,7 @@ impl Plugin for CursorPlugin {
             Update,
             (
                 update_cursor_icon,
-                update_drop_highlights,
+                update_drop_highlights.run_if(resource_changed::<crate::resources::DragState>),
                 update_drop_target_overlays,
             ),
         );