fix(engine): foundation→tableau drag hints, z-lift, and Android battery drain

Fixes #34, #35, #36

- all_hints: add Foundation as source for Tableau hints (guarded by
  take_from_foundation); previously H key never suggested Foundation→Tableau
- end_drag / touch_end_drag: enforce take_from_foundation at input layer
  so a rejected-by-core MoveRequestEvent is never fired
- animation_plugin: pub CARD_ANIM_Z_LIFT so card_plugin can consume it
- update_card_entity: set CardAnim start.z = z + CARD_ANIM_Z_LIFT to
  eliminate 1-frame z artifact where animated card appeared behind resting cards
- solitaire_app: use AutoVsync on Android (caps GPU at display Hz vs
  spinning at 200+ fps); add WinitSettings unfocused reactive_low_power
  so app draws ~1fps when backgrounded

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

solitaire_app/src/lib.rs

@@ -19,6 +19,8 @@ use std::time::{SystemTime, UNIX_EPOCH};
 
 use bevy::prelude::*;
 use bevy::window::{MonitorSelection, PresentMode, WindowPosition};
+#[cfg(target_os = "android")]
+use bevy::winit::{UpdateMode, WinitSettings};
 #[cfg(not(target_os = "android"))]
 use bevy::window::{Monitor, PrimaryMonitor, PrimaryWindow};
 #[cfg(not(target_os = "android"))]
@@ -112,12 +114,22 @@ pub fn run() {
                         name: Some("ferrous-solitaire".into()),
                         resolution: window_resolution,
                         position: window_position,
-                        // AutoNoVsync prefers Mailbox (triple-buffered) and
+                        // On Android, AutoVsync caps the GPU at the display
-                        // falls back to Immediate, eliminating the vsync stall
+                        // refresh rate (~60-90 fps). Without it the renderer
-                        // that AutoVsync produces during continuous window
+                        // spins as fast as the hardware allows, keeping the
-                        // resize on X11 / Wayland. The game's frame budget is
+                        // GPU fully loaded and draining the battery even when
-                        // small enough that a few stray dropped frames from
+                        // the game is completely idle.
-                        // disabling vsync are imperceptible.
+                        //
+                        // On desktop (X11 / Wayland) AutoNoVsync prefers
+                        // Mailbox (triple-buffered) and falls back to
+                        // Immediate, eliminating the vsync stall that
+                        // AutoVsync produces during continuous window resize.
+                        // The game's frame budget is small enough that a few
+                        // stray dropped frames from disabling vsync are
+                        // imperceptible on desktop.
+                        #[cfg(target_os = "android")]
+                        present_mode: PresentMode::AutoVsync,
+                        #[cfg(not(target_os = "android"))]
                         present_mode: PresentMode::AutoNoVsync,
                         // Android windows always fill the screen; max_width/max_height
                         // default to 0.0, which panics Bevy's clamp when min > max.
@@ -204,6 +216,22 @@ pub fn run() {
         .add_plugins(SplashPlugin)
         .add_plugins(DiagnosticsHudPlugin);
 
+    // On Android the default WinitSettings use UpdateMode::Continuous for
+    // the focused window, which means Bevy renders as fast as possible even
+    // when the game is completely idle. Switching to reactive_low_power with
+    // 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.
+    #[cfg(target_os = "android")]
+    app.insert_resource(WinitSettings {
+        focused_mode: UpdateMode::Continuous,
+        unfocused_mode: UpdateMode::reactive_low_power(std::time::Duration::from_secs(1)),
+    });
+
     // Wire the runtime window icon. Bevy 0.18 has no first-class
     // `Window::icon` field; the icon is set through the underlying
     // `winit::window::Window` via `WinitWindows`. Android draws its

solitaire_engine/src/animation_plugin.rs

@@ -81,7 +81,7 @@ const VOLUME_TOAST_SECS: f32 = 1.4;
 ///
 /// 50.0 sits comfortably above the highest pile depth (~1.04) and well below
 /// `DRAG_Z` (500), so a dragged card always renders above an animated one.
-const CARD_ANIM_Z_LIFT: f32 = 50.0;
+pub const CARD_ANIM_Z_LIFT: f32 = 50.0;
 
 /// Per-card stagger interval for the win cascade at Normal speed (seconds).
 ///

solitaire_engine/src/card_plugin.rs

@@ -23,7 +23,7 @@ use solitaire_core::pile::PileType;
 
 use solitaire_core::rules::{can_place_on_foundation, can_place_on_tableau};
 
-use crate::animation_plugin::{CardAnim, EffectiveSlideDuration};
+use crate::animation_plugin::{CardAnim, EffectiveSlideDuration, CARD_ANIM_Z_LIFT};
 use crate::card_animation::CardAnimation;
 use crate::events::{CardFaceRevealedEvent, CardFlippedEvent, StateChangedEvent};
 use crate::game_plugin::GameMutation;
@@ -963,7 +963,12 @@ fn update_card_entity(
     if !has_card_animation {
         // Slide to the new position when it differs meaningfully; snap otherwise.
         if (cur.truncate() - target.truncate()).length() > 1.0 && slide_secs > 0.0 {
-            let start = Vec3::new(cur.x, cur.y, z); // update Z immediately
+            // Lift the card immediately on the first frame of the animation so
+            // it never appears behind a card that is already resting at the
+            // destination slot. `advance_card_anims` will maintain this lift
+            // throughout the tween and snap to `target` (without lift) on
+            // completion.
+            let start = Vec3::new(cur.x, cur.y, z + CARD_ANIM_Z_LIFT);
             commands
                 .entity(entity)
                 .insert(Transform::from_translation(start))

solitaire_engine/src/input_plugin.rs

@@ -734,7 +734,12 @@ fn end_drag(
                                    .is_some_and(|p| can_place_on_foundation(&bottom_card, p))
                     }
                     PileType::Tableau(_) => {
-                        game.0.piles.get(&target)
+                        // Enforce the take-from-foundation rule at the input layer so the
+                        // engine never fires a MoveRequestEvent that game_state would reject.
+                        let foundation_allowed = !matches!(&origin, PileType::Foundation(_))
+                            || game.0.take_from_foundation;
+                        foundation_allowed
+                            && game.0.piles.get(&target)
                                 .is_some_and(|p| can_place_on_tableau(&bottom_card, p))
                     }
                     _ => false,
@@ -988,7 +993,12 @@ fn touch_end_drag(
                                        .is_some_and(|p| can_place_on_foundation(&bottom_card, p))
                         }
                         PileType::Tableau(_) => {
-                            game.0.piles.get(&target)
+                            // Enforce the take-from-foundation rule at the input layer so the
+                            // engine never fires a MoveRequestEvent that game_state would reject.
+                            let foundation_allowed = !matches!(&origin, PileType::Foundation(_))
+                                || game.0.take_from_foundation;
+                            foundation_allowed
+                                && game.0.piles.get(&target)
                                     .is_some_and(|p| can_place_on_tableau(&bottom_card, p))
                         }
                         _ => false,
@@ -1591,6 +1601,26 @@ pub fn all_hints(game: &GameState) -> Vec<(PileType, PileType, usize)> {
         }
     }
 
+    // Pass 2b — Foundation → Tableau moves (only when the rule allows it).
+    // Foundation piles are excluded from Pass 1 & 2's source list because they
+    // should never hint Foundation→Foundation. Here we handle the return path
+    // separately so the guarded `take_from_foundation` rule is respected.
+    if game.take_from_foundation {
+        for slot in 0..4_u8 {
+            let from = PileType::Foundation(slot);
+            let Some(from_pile) = game.piles.get(&from) else { continue };
+            let Some(card) = from_pile.cards.last().filter(|c| c.face_up) else { continue };
+            for i in 0..7_usize {
+                let dest = PileType::Tableau(i);
+                if let Some(dest_pile) = game.piles.get(&dest)
+                    && can_place_on_tableau(card, dest_pile) {
+                        hints.push((from.clone(), dest, 1));
+                        break;
+                    }
+            }
+        }
+    }
+
     // Pass 3 — suggest drawing from the stock when no other hint was found.
     if hints.is_empty() {
         let stock_non_empty = game.piles.get(&PileType::Stock)