fix(engine): eliminate panics, fix dismiss hit-test scope, guard home respawn

CR-2: dismiss_modal_on_scrim_click now queries only the target scrim's
      Children rather than all ModalCard entities globally. Prevents
      dismissing the wrong scrim when two overlapping modals are open.

CR-5: handle_home_draw_mode_buttons and handle_home_difficulty_toggle
      now check other_modal_scrims.is_empty() before the despawn+respawn
      cycle, preventing a concurrent second ModalScrim in the same frame.

H-1:  solitaire_core::game_state — replaced all panicking piles[&key]
      index accesses with safe .get().ok_or(MoveError::InvalidSource)?,
      .get().is_some_and(...), or .get().and_then(...) in draw(),
      check_auto_complete(), next_auto_complete_move(), foundation_slot_for().

H-5:  input_plugin end_drag and touch_end_drag — replaced piles[&target]
      with .get(&target).is_some_and(...) so missing pile types reject the
      move rather than panicking.

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

solitaire_engine/src/home_plugin.rs

@@ -589,6 +589,7 @@ fn handle_home_draw_mode_buttons(
     one_buttons: Query<&Interaction, (With<HomeDrawOneButton>, Changed<Interaction>)>,
     three_buttons: Query<&Interaction, (With<HomeDrawThreeButton>, Changed<Interaction>)>,
     screens: Query<Entity, With<HomeScreen>>,
+    other_modal_scrims: Query<(), (With<crate::ui_modal::ModalScrim>, Without<HomeScreen>)>,
     mut settings: Option<ResMut<SettingsResource>>,
     storage_path: Option<Res<SettingsStoragePath>>,
     mut changed: MessageWriter<SettingsChangedEvent>,
@@ -601,6 +602,12 @@ fn handle_home_draw_mode_buttons(
     if screens.is_empty() {
         return;
     }
+    // Don't respawn while another modal sits on top — the despawn queues
+    // immediately but executes at end of frame, so a respawn in the same
+    // frame would create a second concurrent ModalScrim.
+    if !other_modal_scrims.is_empty() {
+        return;
+    }
     let want_one = one_buttons.iter().any(|i| *i == Interaction::Pressed);
     let want_three = three_buttons.iter().any(|i| *i == Interaction::Pressed);
     if !want_one && !want_three {
@@ -658,6 +665,7 @@ fn handle_home_difficulty_toggle(
     mut commands: Commands,
     toggles: Query<&Interaction, (With<HomeDifficultyToggle>, Changed<Interaction>)>,
     screens: Query<Entity, With<HomeScreen>>,
+    other_modal_scrims: Query<(), (With<crate::ui_modal::ModalScrim>, Without<HomeScreen>)>,
     mut diff_expanded: ResMut<DifficultyExpanded>,
     progress: Option<Res<ProgressResource>>,
     stats: Option<Res<StatsResource>>,
@@ -668,6 +676,9 @@ fn handle_home_difficulty_toggle(
     if screens.is_empty() {
         return;
     }
+    if !other_modal_scrims.is_empty() {
+        return;
+    }
     if !toggles.iter().any(|i| *i == Interaction::Pressed) {
         return;
     }

solitaire_engine/src/input_plugin.rs

@@ -717,13 +717,12 @@ fn end_drag(
                 let ok = match &target {
                     PileType::Foundation(_) => {
                         count == 1
-                            && can_place_on_foundation(
-                                &bottom_card,
-                                &game.0.piles[&target],
-                            )
+                            && game.0.piles.get(&target)
+                                   .is_some_and(|p| can_place_on_foundation(&bottom_card, p))
                     }
                     PileType::Tableau(_) => {
-                        can_place_on_tableau(&bottom_card, &game.0.piles[&target])
+                        game.0.piles.get(&target)
+                            .is_some_and(|p| can_place_on_tableau(&bottom_card, p))
                     }
                     _ => false,
                 };
@@ -972,10 +971,12 @@ fn touch_end_drag(
                     let ok = match &target {
                         PileType::Foundation(_) => {
                             count == 1
-                                && can_place_on_foundation(&bottom_card, &game.0.piles[&target])
+                                && game.0.piles.get(&target)
+                                       .is_some_and(|p| can_place_on_foundation(&bottom_card, p))
                         }
                         PileType::Tableau(_) => {
-                            can_place_on_tableau(&bottom_card, &game.0.piles[&target])
+                            game.0.piles.get(&target)
+                                .is_some_and(|p| can_place_on_tableau(&bottom_card, p))
                         }
                         _ => false,
                     };

solitaire_engine/src/ui_modal.rs

@@ -603,7 +603,7 @@ pub fn dismiss_modal_on_scrim_click(
     mut commands: Commands,
     mouse: Option<Res<ButtonInput<MouseButton>>>,
     windows: Query<&Window, With<PrimaryWindow>>,
-    scrims: Query<Entity, (With<ModalScrim>, With<ScrimDismissible>)>,
+    scrims: Query<(Entity, &Children), (With<ModalScrim>, With<ScrimDismissible>)>,
     cards: Query<(&UiGlobalTransform, &ComputedNode), With<ModalCard>>,
 ) {
     let Some(mouse) = mouse else { return };
@@ -620,15 +620,19 @@ pub fn dismiss_modal_on_scrim_click(
     // Topmost-only: bail after the first dismissible scrim. Stacked
     // dismissible modals are not currently a real case, but this guard
     // keeps the behaviour predictable if they ever arise.
-    let Some(scrim_entity) = scrims.iter().next() else {
+    let Some((scrim_entity, scrim_children)) = scrims.iter().next() else {
         return;
     };
 
-    let cursor_over_card = cards.iter().any(|(transform, computed)| {
-        let inv = computed.inverse_scale_factor;
-        let size_logical = computed.size() * inv;
-        let centre_logical = transform.translation * inv;
-        cursor_is_inside_rect(cursor, centre_logical, size_logical)
+    // Only test the ModalCard(s) that belong to THIS scrim, not cards
+    // from any other concurrently-open modal.
+    let cursor_over_card = scrim_children.iter().any(|child| {
+        cards.get(child).is_ok_and(|(transform, computed)| {
+            let inv = computed.inverse_scale_factor;
+            let size_logical = computed.size() * inv;
+            let centre_logical = transform.translation * inv;
+            cursor_is_inside_rect(cursor, centre_logical, size_logical)
+        })
     });
 
     if !cursor_over_card {