fix(engine): ensure dragged card stack z-order is above all piles (closes #35)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/input_plugin.rs

@@ -64,6 +64,16 @@ pub enum TouchDragSet {
 
 /// Z-depth used for cards while being dragged — above all resting cards.
 const DRAG_Z: f32 = 500.0;
+/// Relative Z step between cards inside a dragged stack.
+///
+/// Must stay at least as large as [`STACK_FAN_FRAC`], otherwise Android's
+/// per-card corner overlay children (`local_z = 0.02`) can bleed above the
+/// card body stacked directly above them while dragging.
+const DRAG_STACK_Z_STEP: f32 = STACK_FAN_FRAC;
+
+fn dragged_card_z(index: usize) -> f32 {
+    DRAG_Z + index as f32 * DRAG_STACK_Z_STEP
+}
 
 /// Solver budgets used by the H-key hint system.
 ///
@@ -638,7 +648,7 @@ fn follow_drag(
             if let Some((_, mut transform, mut sprite)) =
                 card_transforms.iter_mut().find(|(ce, _, _)| ce.card_id == id)
             {
-                transform.translation.z = DRAG_Z + i as f32 * 0.01;
+                transform.translation.z = dragged_card_z(i);
                 sprite.color.set_alpha(0.85);
             }
         }
@@ -904,7 +914,7 @@ fn touch_follow_drag(
             if let Some((_, mut transform, mut sprite)) =
                 card_transforms.iter_mut().find(|(ce, _, _)| ce.card_id == id)
             {
-                transform.translation.z = DRAG_Z + i as f32 * 0.01;
+                transform.translation.z = dragged_card_z(i);
                 sprite.color.set_alpha(0.85);
             }
         }
@@ -1659,6 +1669,17 @@ mod tests {
     use crate::layout::compute_layout;
     use solitaire_core::game_state::{DrawMode, GameState};
 
+    #[test]
+    fn dragged_card_z_matches_resting_stack_step() {
+        assert!((dragged_card_z(0) - DRAG_Z).abs() < 1e-6);
+        let step = dragged_card_z(1) - dragged_card_z(0);
+        assert!(step > 0.02, "drag step must exceed Android overlay local_z, got {step}");
+        assert!(
+            step + 1e-4 >= STACK_FAN_FRAC,
+            "drag step must stay aligned with resting stack spacing, got {step}"
+        );
+    }
+
     #[test]
     fn point_in_rect_inside_returns_true() {
         let center = Vec2::new(10.0, 20.0);