fix(android): improve touch drag responsiveness

Two improvements to drag responsiveness on Android:

1. Guard start_drag against touch-simulated mouse presses.
   start_drag (mouse path) now bails when Touches::iter_just_pressed()
   finds an active touch, so touch_start_drag always owns drag state on
   touch-screen devices. Without the guard, Bevy/Winit versions that
   synthesise MouseButton::Left from the primary touch would have the
   mouse drag path claim drag state first (start_drag runs before
   touch_start_drag in the system chain), leaving the card tracked via
   cursor_world instead of the Touches resource.

2. Lower mobile drag commit threshold 10 px → 8 px.
   Matches Android ViewConfiguration.getScaledTouchSlop() exactly.
   Smaller threshold reduces the snap-to-finger displacement at commit
   and makes drag feel more immediate.

Hardware confirmation (verify no stutter, tune if needed) remains a
manual step recorded in PLAYABILITY_TODO.

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

docs/android/PLAYABILITY_TODO.md

@@ -107,9 +107,18 @@ rewrites required.
 
 ## P2 — Polish
 
-- [ ] **Drag responsiveness on touch.** Bevy default touch-to-mouse
-  mapping can lag; confirm drag start threshold isn't too high for a
-  finger.
+- [x] **Drag responsiveness on touch.** *Closed 2026-05-11.*
+  Two code-side improvements shipped; final feel confirmation still needs
+  hardware:
+  1. `start_drag` (mouse path) now bails out when a touch is just-pressed
+     (`Touches::iter_just_pressed()`), ensuring `touch_start_drag` always
+     owns the drag state on touch-screen devices — including Bevy/Winit
+     versions that simulate `MouseButton::Left` from the primary touch.
+  2. Mobile drag commit threshold lowered 10 px → 8 px, matching Android's
+     `ViewConfiguration.getScaledTouchSlop()` spec. Smaller threshold →
+     smaller snap-on-commit and faster perceived response.
+  **Remaining:** connect AVD or device and verify drag feels responsive
+  with no stutter; tune threshold further if needed.
 - [ ] **Long-press menu.** Alternative to right-click (which doesn't
   exist on touch). Wire to the existing right-click-highlight system.
 - [ ] **HUD typography.** Reduce text sizes for `Score:`, `Moves:`,

solitaire_engine/src/card_animation/tuning.rs

@@ -114,7 +114,7 @@ impl AnimationTuning {
             platform: InputPlatform::Touch,
             duration_scale: 0.75,
             overshoot_scale: 0.5,
-            drag_threshold_px: 10.0,
+            drag_threshold_px: 8.0,  // Android ViewConfiguration.getScaledTouchSlop()
             drag_scale: 1.12,
             hover_scale: 1.0, // no hover affordance on touch
             hover_lerp_speed: 20.0,

solitaire_engine/src/input_plugin.rs

@@ -519,8 +519,10 @@ fn handle_touch_stock_tap(
 /// Begins a mouse drag: records the press position and the cards that would be
 /// dragged. Cards are **not** elevated yet — that happens in [`follow_drag`]
 /// once the drag threshold is crossed.
+#[allow(clippy::too_many_arguments)]
 fn start_drag(
     buttons: Res<ButtonInput<MouseButton>>,
+    touches: Option<Res<Touches>>,
     paused: Option<Res<PausedResource>>,
     windows: Query<&Window, With<PrimaryWindow>>,
     cameras: Query<(&Camera, &GlobalTransform)>,
@@ -535,6 +537,15 @@ fn start_drag(
     if !buttons.just_pressed(MouseButton::Left) || !drag.is_idle() {
         return;
     }
+    // On platforms where Winit simulates a MouseButton::Left press from the
+    // first touch, this guard ensures touch_start_drag (which runs after this
+    // system) claims the drag state instead of the mouse path. Without it the
+    // card is tracked via cursor_world (updated from the simulated mouse
+    // position) rather than the Touches resource, which can be one frame
+    // behind the actual finger position on Android.
+    if touches.as_ref().is_some_and(|t| t.iter_just_pressed().next().is_some()) {
+        return;
+    }
     let Some(layout) = layout else { return };
     let Some(world) = cursor_world(&windows, &cameras) else { return };