fix(engine): swallow game-mutating hotkeys during pointer drags

U (undo), N (new game), Z (zen), D/Space (draw) and G (forfeit) were
not gated on an active drag, so pressing one mid-drag fired a
StateChangedEvent whose card re-sync inserted a `CardAnim` on the
dragged cards — fighting `follow_drag`'s per-frame Transform writes
(jittering XY, animated Z) and leaving `DragState` origin indices
stale against the mutated state.

`handle_keyboard_core` and `handle_keyboard_forfeit` now reuse the
mutual-exclusion guard `handle_selection_keys` already documents: skip
while `DragState` is non-idle, unless it is the keyboard-lift sentinel
(`KEYBOARD_DRAG_TOUCH_ID`), whose lift is already dropped cleanly by
`clear_selection_on_state_change`.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/input_plugin/tests.rs

@@ -817,3 +817,123 @@ fn pressing_h_spawns_pending_hint_task() {
         "pressing H must spawn an async hint task",
     );
 }
+
+// -----------------------------------------------------------------------
+// Pointer-drag / keyboard mutual exclusion.
+//
+// A state mutation fired while `follow_drag` is writing the dragged cards'
+// transforms every frame would make the card re-sync insert a `CardAnim`
+// on the same entities (two systems fighting over `Transform`) and leave
+// `DragState`'s origin indices stale. `handle_keyboard_core` and
+// `handle_keyboard_forfeit` therefore swallow game-mutating shortcuts
+// while a mouse or touch drag is live, mirroring `handle_selection_keys`.
+// The keyboard-lift sentinel is exempt: `clear_selection_on_state_change`
+// drops that lift cleanly when the state moves.
+// -----------------------------------------------------------------------
+
+fn keyboard_core_app() -> App {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins);
+    app.add_message::<UndoRequestEvent>();
+    app.add_message::<NewGameRequestEvent>();
+    app.add_message::<InfoToastEvent>();
+    app.add_message::<DrawRequestEvent>();
+    app.add_message::<StartZenRequestEvent>();
+    app.init_resource::<ButtonInput<KeyCode>>();
+    app.init_resource::<DragState>();
+    app.add_systems(Update, handle_keyboard_core);
+    app
+}
+
+fn press_key(app: &mut App, key: KeyCode) {
+    let mut input = app.world_mut().resource_mut::<ButtonInput<KeyCode>>();
+    input.release(key);
+    input.clear();
+    input.press(key);
+}
+
+fn message_count<M: Message>(app: &App) -> usize {
+    let messages = app.world().resource::<Messages<M>>();
+    let mut cursor = messages.get_cursor();
+    cursor.read(messages).count()
+}
+
+fn set_drag(app: &mut App, active_touch_id: Option<u64>) {
+    let mut drag = app.world_mut().resource_mut::<DragState>();
+    drag.cards = vec![Card::new(Deck::Deck1, Suit::Clubs, Rank::Two)];
+    drag.committed = true;
+    drag.active_touch_id = active_touch_id;
+}
+
+#[test]
+fn u_key_fires_undo_when_no_drag_active() {
+    let mut app = keyboard_core_app();
+    press_key(&mut app, KeyCode::KeyU);
+    app.update();
+    assert_eq!(
+        message_count::<UndoRequestEvent>(&app),
+        1,
+        "U with an idle DragState must fire UndoRequestEvent",
+    );
+}
+
+#[test]
+fn u_key_ignored_during_committed_mouse_drag() {
+    let mut app = keyboard_core_app();
+    set_drag(&mut app, None); // mouse drag
+    press_key(&mut app, KeyCode::KeyU);
+    app.update();
+    assert_eq!(
+        message_count::<UndoRequestEvent>(&app),
+        0,
+        "U during a mouse drag must be swallowed",
+    );
+}
+
+#[test]
+fn draw_key_ignored_during_touch_drag() {
+    let mut app = keyboard_core_app();
+    set_drag(&mut app, Some(7)); // real touch id
+    press_key(&mut app, KeyCode::KeyD);
+    app.update();
+    assert_eq!(
+        message_count::<DrawRequestEvent>(&app),
+        0,
+        "D during a touch drag must be swallowed",
+    );
+}
+
+#[test]
+fn u_key_allowed_during_keyboard_lift() {
+    let mut app = keyboard_core_app();
+    set_drag(
+        &mut app,
+        Some(crate::selection_plugin::KEYBOARD_DRAG_TOUCH_ID),
+    );
+    press_key(&mut app, KeyCode::KeyU);
+    app.update();
+    assert_eq!(
+        message_count::<UndoRequestEvent>(&app),
+        1,
+        "the keyboard-lift sentinel must not block core shortcuts",
+    );
+}
+
+#[test]
+fn g_key_forfeit_ignored_during_mouse_drag() {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins);
+    app.add_message::<ForfeitRequestEvent>();
+    app.init_resource::<ButtonInput<KeyCode>>();
+    app.init_resource::<DragState>();
+    app.add_systems(Update, handle_keyboard_forfeit);
+
+    set_drag(&mut app, None);
+    press_key(&mut app, KeyCode::KeyG);
+    app.update();
+    assert_eq!(
+        message_count::<ForfeitRequestEvent>(&app),
+        0,
+        "G during a mouse drag must be swallowed",
+    );
+}