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/animation_plugin.rs

@@ -454,8 +454,8 @@ fn handle_settings_toast(
     for ev in events.read() {
         let sfx = ev.0.sfx_volume;
         let music = ev.0.music_volume;
-        let sfx_changed = last_sfx.is_none_or(|prev| (prev - sfx).abs() > f32::EPSILON);
-        let music_changed = last_music.is_none_or(|prev| (prev - music).abs() > f32::EPSILON);
+        let sfx_changed = last_sfx.is_none_or(|prev| (prev - sfx).abs() > 0.001);
+        let music_changed = last_music.is_none_or(|prev| (prev - music).abs() > 0.001);
         *last_sfx = Some(sfx);
         *last_music = Some(music);
         if sfx_changed {

solitaire_engine/src/card_plugin.rs

@@ -451,7 +451,9 @@ impl Plugin for CardPlugin {
                     clear_right_click_highlights_on_state_change.after(GameMutation),
                     clear_right_click_highlights_on_pause,
                     update_stock_empty_indicator.after(GameMutation),
-                    update_stock_count_badge.after(GameMutation),
+                    update_stock_count_badge
+                        .after(GameMutation)
+                        .run_if(resource_changed::<crate::GameStateResource>),
                     collect_resize_events.after(LayoutSystem::UpdateOnResize),
                     snap_cards_on_window_resize.after(collect_resize_events),
                 ),

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,
             ),
         );

solitaire_engine/src/game_plugin.rs

@@ -1079,9 +1079,7 @@ fn check_no_moves(
 ) {
     // Reset the debounce flag on every state change so if something changes
     // we re-evaluate on the next state change.
-    let had_event = events.read().next().is_some();
-    // Drain remaining events to avoid leaking.
-    events.clear();
+    let had_event = events.read().count() > 0;
 
     if !had_event {
         return;

solitaire_engine/src/hud_plugin.rs

@@ -417,7 +417,13 @@ impl Plugin for HudPlugin {
             .add_systems(Update, (update_hud_avatar, handle_avatar_button))
             .add_systems(Update, update_won_previously.after(GameMutation))
             .add_systems(Update, announce_auto_complete.after(GameMutation))
-            .add_systems(Update, update_selection_hud)
+            .add_systems(
+                Update,
+                update_selection_hud.run_if(
+                    resource_exists_and_changed::<SelectionState>
+                        .or(resource_exists_and_changed::<GameStateResource>),
+                ),
+            )
             .add_systems(Update, update_hud_typography)
             .add_systems(
                 Update,

solitaire_engine/src/input_plugin.rs

@@ -1162,7 +1162,9 @@ fn find_draggable_at(
                 (i, pile_cards.cards.len())
             } else {
                 if i != pile_cards.cards.len() - 1 {
-                    return None;
+                    // Non-top card on a non-tableau pile — not draggable; skip
+                    // this pile and continue searching remaining piles.
+                    break;
                 }
                 (i, i + 1)
             };

solitaire_engine/src/settings_plugin.rs

@@ -401,8 +401,10 @@ impl Plugin for SettingsPlugin {
                     update_anim_speed_text,
                     update_color_blind_text,
                     update_high_contrast_text,
-                    update_high_contrast_borders,
-                    update_high_contrast_backgrounds,
+                    update_high_contrast_borders
+                        .run_if(resource_changed::<SettingsResource>),
+                    update_high_contrast_backgrounds
+                        .run_if(resource_changed::<SettingsResource>),
                     update_reduce_motion_text,
                     update_tooltip_delay_text,
                     update_time_bonus_multiplier_text,