feat(engine): reactive render — animations drive RequestRedraw, focused_mode reactive on Android

All per-frame animation tick systems now write MessageWriter<RequestRedraw>
each frame they have active work, allowing WinitSettings focused_mode to
switch from Continuous to reactive_low_power(100 ms) on Android.

Systems updated:
- advance_card_animations (CardAnimationPlugin)
- advance_card_anims (AnimationPlugin — deal/win cascade)
- tick_shake_anim, tick_settle_anim, tick_foundation_flourish (FeedbackAnimPlugin)
- drive_toast_display (AnimationPlugin — toast countdown)
- drive_auto_complete (AutoCompletePlugin — step interval keepalive)

The 100 ms low-power ceiling means the game timer still ticks ~10×/s
with no input; animations self-sustain via the redraw chain at full
frame rate while active; and the GPU is completely idle between frames
when the board is static.

Each plugin registers add_message::<RequestRedraw>() so the message
type is available under MinimalPlugins in unit tests.

Closes #78, #79

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

solitaire_app/src/lib.rs

@@ -172,13 +172,16 @@ fn build_app_with_settings(
     // a 1-second ceiling when the app is backgrounded cuts wake-up frequency
     // from ~60 Hz to ≤1 Hz, dramatically reducing background battery drain.
     //
-    // The focused mode stays Continuous so that card-slide animations remain
-    // smooth. PresentMode::AutoVsync (set above) keeps the GPU capped at the
-    // display refresh rate (~60 Hz) when foregrounded, which already prevents
-    // the GPU from spinning at 200+ fps between vsync intervals.
+    // focused_mode uses reactive_low_power(100 ms) so the CPU only wakes when
+    // an event arrives (touch, resize, etc.) or an animation system writes
+    // RequestRedraw. The 100 ms ceiling is a fallback that ensures the game
+    // timer ticks at least 10×/s even with no input, while keeping the GPU
+    // completely idle between frames when the board is static.
+    // PresentMode::AutoVsync (set above) still caps the GPU at the display
+    // refresh rate when frames do render.
     #[cfg(target_os = "android")]
     app.insert_resource(WinitSettings {
-        focused_mode: UpdateMode::Continuous,
+        focused_mode: UpdateMode::reactive_low_power(std::time::Duration::from_millis(100)),
         unfocused_mode: UpdateMode::reactive_low_power(std::time::Duration::from_secs(1)),
     });