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>

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,