feat(engine): duck the glass tab bar during card drags

While a drag is committed the floating bar slides below the screen
edge (UiTransform translation - no layout dirtying, no fight with the
safe-area anchor over Node::bottom) so it never occludes a drop target
on the bottom tableau rows; it slides back on release. Same tween rate
as the label reveal, snaps under reduce-motion. Desktop docked bar is
untouched (no TabBarDuck component).

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

solitaire_engine/src/hud_plugin/tests.rs

@@ -1014,6 +1014,54 @@ fn expansion_width_is_linear_in_progress() {
     assert_eq!(expansion_width(4, 16.0, 1.5), full);
 }
 
+/// While a committed drag is active the bar must slide down (positive
+/// `UiTransform` y — off-screen) and return to identity once the drag
+/// ends. Manual time steps make the tween deterministic.
+#[test]
+fn tab_bar_ducks_during_committed_drag_and_returns() {
+    use crate::resources::DragState;
+    use bevy::ui::UiTransform;
+
+    let mut app = bar_only_app(true);
+    set_manual_time_step(&mut app, 0.05);
+    app.add_systems(Update, duck_tab_bar_during_drag);
+    app.insert_resource(DragState {
+        committed: true,
+        ..default()
+    });
+
+    let translation_y_px = |app: &mut App| -> f32 {
+        let world = app.world_mut();
+        let mut q = world.query_filtered::<&UiTransform, With<HudActionBar>>();
+        let t = q.single(world).expect("bar carries UiTransform");
+        match t.translation.y {
+            Val::Px(px) => px,
+            other => panic!("duck must write Val::Px, got {other:?}"),
+        }
+    };
+
+    // 0.18s slide at 0.05s/frame: fully ducked within 5 frames.
+    app.update();
+    app.update();
+    let mid = translation_y_px(&mut app);
+    assert!(mid > 0.0, "bar must start sliding down: {mid}");
+    for _ in 0..5 {
+        app.update();
+    }
+    let ducked = translation_y_px(&mut app);
+    assert!(
+        ducked > TAB_BAR_CLEARANCE_PX,
+        "fully ducked bar must clear its own floating footprint: {ducked}"
+    );
+
+    // Drag ends → bar slides back to identity.
+    app.world_mut().resource_mut::<DragState>().committed = false;
+    for _ in 0..8 {
+        app.update();
+    }
+    assert_eq!(translation_y_px(&mut app), 0.0);
+}
+
 /// The tween must move at the `MOTION_SLIDE_SECS` rate, never overshoot,
 /// and snap instantly under reduce-motion.
 #[test]