feat(engine): Android UX sweep — tap-to-move, safe area, HUD polish

Single-tap auto-move (input_plugin):
- Remove 0.5 s double-tap window; any uncommitted TouchPhase::Ended on
  a face-up card now fires MoveRequestEvent immediately.

Bottom safe-area inset (layout, table_plugin):
- compute_layout gains safe_area_bottom param; height budget and bottom
  margin both respect the navigation bar reservation.

Card back contrast (card_plugin):
- CardBackFrame child sprite (gray, card_size + 3 px, local z=-0.01)
  spawned behind every face-down card so the dark back_0.png reads as
  a distinct rectangle against the dark felt.

HUD action bar compactness (hud_plugin):
- max_width 50% → 65% on the action button row; 6 buttons now wrap to
  2 rows instead of 3 on a 360 dp phone.

Dynamic tableau fan fraction (layout, card_plugin):
- Layout gains available_tableau_height field.
- update_tableau_fan_frac system (after GameMutation, before
  sync_cards_on_change) grows face-up fan from 0.25 to the window max
  as revealed column depth increases. Face-down fan is left at the
  window-adaptive value so stacks stay visible.

ModesPopover + MenuPopover light-dismiss (hud_plugin):
- Fullscreen transparent Button backdrop spawned at Z_HUD+4 behind each
  popover; tapping outside the panel despawns both panel and backdrop.

Stock badge legibility (card_plugin):
- Badge font TYPE_CAPTION (11 pt) → TYPE_BODY (14 pt); background
  sprite 28×16 → 34×20 world units.

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

solitaire_engine/src/cursor_plugin.rs

@@ -604,7 +604,7 @@ mod tests {
         use crate::layout::compute_layout;
 
         let game = GameState::new(42, DrawMode::DrawOne);
-        let layout = compute_layout(Vec2::new(1280.0, 800.0), 0.0);
+        let layout = compute_layout(Vec2::new(1280.0, 800.0), 0.0, 0.0);
         // A cursor far off-screen should never hit anything.
         assert!(!cursor_over_draggable(Vec2::new(-9999.0, -9999.0), &game, &layout));
     }
@@ -624,7 +624,7 @@ mod tests {
         let mut app = App::new();
         app.add_plugins(MinimalPlugins)
             .insert_resource(GameStateResource(game))
-            .insert_resource(LayoutResource(compute_layout(Vec2::new(1280.0, 800.0), 0.0)))
+            .insert_resource(LayoutResource(compute_layout(Vec2::new(1280.0, 800.0), 0.0, 0.0)))
             .insert_resource(DragState::default())
             .add_systems(Update, update_drop_target_overlays);
         app