feat(engine): hint ghost-motion preview (Phase H)

A hint now spawns a translucent copy of the hinted card that glides to
the suggested destination twice (0.7s per pass, SmoothSnap easing, tail
fade so the loop reads as a repeat), alongside the existing static
source/destination highlights. Auto-disabled under reduce-motion — the
static highlights remain the whole story. The ghost despawns on timer,
on a fresh hint, or the moment the board changes (a preview of a stale
board is worse than none).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/feedback_anim_plugin.rs

@@ -49,10 +49,11 @@ use solitaire_core::klondike_adapter::foundation_from_slot;
 use solitaire_data::AnimSpeed;
 
 use crate::animation_plugin::CardAnim;
-use crate::card_plugin::CardEntity;
+use crate::card_animation::{MotionCurve, sample_curve};
+use crate::card_plugin::{CardEntity, CardEntityIndex};
 use crate::events::{
-    DrawRequestEvent, FoundationCompletedEvent, MoveRejectedEvent, MoveRequestEvent,
-    NewGameRequestEvent,
+    DrawRequestEvent, FoundationCompletedEvent, HintVisualEvent, MoveRejectedEvent,
+    MoveRequestEvent, NewGameRequestEvent, StateChangedEvent,
 };
 use crate::game_plugin::GameMutation;
 use crate::layout::LayoutResource;
@@ -207,6 +208,8 @@ impl Plugin for FeedbackAnimPlugin {
             .add_message::<MoveRejectedEvent>()
             .add_message::<NewGameRequestEvent>()
             .add_message::<FoundationCompletedEvent>()
+            .add_message::<HintVisualEvent>()
+            .add_message::<StateChangedEvent>()
             .add_message::<RequestRedraw>()
             .add_systems(
                 Update,
@@ -224,6 +227,20 @@ impl Plugin for FeedbackAnimPlugin {
                     start_deal_anim.after(GameMutation),
                     start_foundation_flourish.after(GameMutation),
                 ),
+            )
+            // Hint ghost (Phase H): the spawn reads card Transform/Sprite,
+            // so it orders after the board painters; the tick only touches
+            // ghost entities (Without<CardEntity>) and stays conflict-free.
+            .add_systems(
+                Update,
+                (
+                    spawn_hint_ghost
+                        .after(GameMutation)
+                        .after(crate::card_plugin::BoardVisuals),
+                    tick_hint_ghosts,
+                    despawn_hint_ghosts_on_state_change.after(GameMutation),
+                )
+                    .chain(),
             );
     }
 }
@@ -664,6 +681,147 @@ fn pile_cards(
     }
 }
 
+// ---------------------------------------------------------------------------
+// Phase H — hint ghost-motion preview
+// ---------------------------------------------------------------------------
+
+/// Duration of one ghost glide from the hinted card to its destination.
+const HINT_GHOST_PASS_SECS: f32 = 0.7;
+/// How many glides one hint plays before the ghost despawns. Two reads
+/// as "this move, over there" without outstaying the 2 s static
+/// highlight it accompanies.
+const HINT_GHOST_PASSES: f32 = 2.0;
+/// Ghost translucency — clearly a projection, never mistakable for the
+/// real card.
+const HINT_GHOST_ALPHA: f32 = 0.45;
+/// Ghost render depth: above every settled pile (~1.04 max) and the
+/// in-flight `CardAnim` lift (50), below a dragged card (500).
+const HINT_GHOST_Z: f32 = 400.0;
+
+/// A translucent copy of the hinted card gliding to the suggested
+/// destination (Phase H). Purely decorative — despawned by timer, by a
+/// newer hint, or by any state change.
+#[derive(Component, Debug)]
+pub struct HintGhost {
+    start: Vec3,
+    target: Vec3,
+    elapsed: f32,
+}
+
+/// Normalised progress of the current glide pass, restarting from the
+/// source each pass. Pure for unit testing.
+fn hint_ghost_pass_t(elapsed: f32) -> f32 {
+    (elapsed % HINT_GHOST_PASS_SECS) / HINT_GHOST_PASS_SECS
+}
+
+/// Ghost alpha at `pass_t` — full strength for most of the glide, then
+/// fading over the last 20 % so the loop restart reads as a repeat
+/// rather than a teleport. Pure for unit testing.
+fn hint_ghost_alpha(pass_t: f32) -> f32 {
+    let fade_in_tail = ((pass_t - 0.8) / 0.2).clamp(0.0, 1.0);
+    HINT_GHOST_ALPHA * (1.0 - fade_in_tail)
+}
+
+/// Spawns the ghost when a hint fires. The static highlights (source
+/// card + gold destination pile) still spawn regardless; under
+/// reduce-motion they are the whole story and no ghost appears
+/// (`design-system.md` §Accessibility).
+#[allow(clippy::too_many_arguments)]
+fn spawn_hint_ghost(
+    mut events: MessageReader<HintVisualEvent>,
+    settings: Option<Res<SettingsResource>>,
+    index: Option<Res<CardEntityIndex>>,
+    layout: Option<Res<LayoutResource>>,
+    cards: Query<(&Transform, &Sprite), With<CardEntity>>,
+    existing: Query<Entity, With<HintGhost>>,
+    mut commands: Commands,
+) {
+    if events.is_empty() {
+        return;
+    }
+    if settings.is_some_and(|s| s.0.reduce_motion_mode) {
+        events.clear();
+        return;
+    }
+    let (Some(index), Some(layout)) = (index, layout) else {
+        events.clear();
+        return;
+    };
+    for ev in events.read() {
+        // A fresh hint replaces any ghost still in flight.
+        for entity in &existing {
+            commands.entity(entity).despawn();
+        }
+        let Some(card_entity) = index.get(&ev.source_card) else {
+            continue;
+        };
+        let Ok((transform, sprite)) = cards.get(card_entity) else {
+            continue;
+        };
+        let Some(&dest) = layout.0.pile_positions.get(&ev.dest_pile) else {
+            continue;
+        };
+        let start = transform.translation.truncate().extend(HINT_GHOST_Z);
+        let mut ghost_sprite = sprite.clone();
+        ghost_sprite.color = ghost_sprite.color.with_alpha(HINT_GHOST_ALPHA);
+        commands.spawn((
+            HintGhost {
+                start,
+                target: dest.extend(HINT_GHOST_Z),
+                elapsed: 0.0,
+            },
+            ghost_sprite,
+            Transform::from_translation(start),
+        ));
+    }
+}
+
+/// Advances every ghost: eased glide per pass, tail fade, despawn after
+/// [`HINT_GHOST_PASSES`]. Frozen while paused, like every other
+/// decorative animation.
+#[allow(clippy::type_complexity)]
+fn tick_hint_ghosts(
+    time: Res<Time>,
+    paused: Option<Res<PausedResource>>,
+    mut ghosts: Query<
+        (Entity, &mut HintGhost, &mut Transform, &mut Sprite),
+        (Without<CardEntity>, Without<PileMarker>),
+    >,
+    mut commands: Commands,
+) {
+    if paused.is_some_and(|p| p.0) {
+        return;
+    }
+    let dt = time.delta_secs();
+    for (entity, mut ghost, mut transform, mut sprite) in ghosts.iter_mut() {
+        ghost.elapsed += dt;
+        if ghost.elapsed >= HINT_GHOST_PASS_SECS * HINT_GHOST_PASSES {
+            commands.entity(entity).despawn();
+            continue;
+        }
+        let pass_t = hint_ghost_pass_t(ghost.elapsed);
+        let eased = sample_curve(MotionCurve::SmoothSnap, pass_t);
+        transform.translation = ghost.start.lerp(ghost.target, eased);
+        sprite.color = sprite.color.with_alpha(hint_ghost_alpha(pass_t));
+    }
+}
+
+/// A ghost previews a move against the *current* board; the moment the
+/// board changes (move, undo, draw, new game) it is stale and vanishes.
+fn despawn_hint_ghosts_on_state_change(
+    mut events: MessageReader<StateChangedEvent>,
+    ghosts: Query<Entity, With<HintGhost>>,
+    mut commands: Commands,
+) {
+    if events.is_empty() {
+        return;
+    }
+    events.clear();
+    for entity in &ghosts {
+        commands.entity(entity).despawn();
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Unit tests (pure functions only — no Bevy world required)
 // ---------------------------------------------------------------------------
@@ -955,4 +1113,147 @@ mod tests {
             "FoundationFlourish must not be inserted under reduce-motion"
         );
     }
+
+    // -----------------------------------------------------------------------
+    // Phase H — hint ghost-motion preview
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn hint_ghost_pass_t_restarts_each_pass() {
+        assert_eq!(hint_ghost_pass_t(0.0), 0.0);
+        // Just past one full pass, progress wraps back near zero.
+        assert!(hint_ghost_pass_t(HINT_GHOST_PASS_SECS + 0.01) < 0.1);
+    }
+
+    #[test]
+    fn hint_ghost_alpha_fades_only_in_the_tail() {
+        assert_eq!(hint_ghost_alpha(0.0), HINT_GHOST_ALPHA);
+        assert_eq!(hint_ghost_alpha(0.79), HINT_GHOST_ALPHA);
+        assert!(hint_ghost_alpha(0.9) < HINT_GHOST_ALPHA);
+        assert!(hint_ghost_alpha(1.0).abs() < 1e-6);
+    }
+
+    /// App with one indexed card entity and a real layout, ready to
+    /// receive `HintVisualEvent`s.
+    fn ghost_app(reduce_motion: bool) -> (App, Card) {
+        use solitaire_core::{Deck, Rank, Suit};
+
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins)
+            .add_plugins(FeedbackAnimPlugin);
+        // Sibling systems in the plugin take GameStateResource
+        // non-optionally; give them a fresh deal.
+        app.insert_resource(GameStateResource(
+            solitaire_core::game_state::GameState::new(1, solitaire_core::DrawStockConfig::DrawOne),
+        ));
+        app.insert_resource(SettingsResource(solitaire_data::Settings {
+            reduce_motion_mode: reduce_motion,
+            ..Default::default()
+        }));
+
+        let card = Card::new(Deck::Deck1, Suit::Spades, Rank::Ace);
+        let entity = app
+            .world_mut()
+            .spawn((
+                CardEntity { card: card.clone() },
+                Sprite::default(),
+                Transform::from_xyz(-100.0, 40.0, 1.0),
+            ))
+            .id();
+        let mut index = CardEntityIndex::default();
+        index.0.insert(card.clone(), entity);
+        app.insert_resource(index);
+        app.insert_resource(LayoutResource(crate::layout::compute_layout(
+            Vec2::new(1280.0, 720.0),
+            0.0,
+            0.0,
+            true,
+        )));
+        app.update();
+        (app, card)
+    }
+
+    fn fire_hint(app: &mut App, card: Card) {
+        use solitaire_core::Tableau;
+        app.world_mut().write_message(HintVisualEvent {
+            source_card: card,
+            dest_pile: KlondikePile::Tableau(Tableau::Tableau2),
+        });
+        app.update();
+    }
+
+    fn ghost_count(app: &mut App) -> usize {
+        app.world_mut()
+            .query::<&HintGhost>()
+            .iter(app.world())
+            .count()
+    }
+
+    #[test]
+    fn hint_event_spawns_one_ghost_at_the_source_card() {
+        let (mut app, card) = ghost_app(false);
+        fire_hint(&mut app, card);
+
+        assert_eq!(ghost_count(&mut app), 1, "hint must spawn one ghost");
+        let transform = app
+            .world_mut()
+            .query_filtered::<&Transform, With<HintGhost>>()
+            .single(app.world())
+            .expect("ghost transform");
+        // The tick system may have advanced the ghost by one real-time
+        // frame already; assert it is still essentially at the source.
+        assert!(
+            transform
+                .translation
+                .truncate()
+                .distance(Vec2::new(-100.0, 40.0))
+                < 5.0,
+            "ghost must start at the hinted card, got {:?}",
+            transform.translation
+        );
+        assert_eq!(transform.translation.z, HINT_GHOST_Z);
+    }
+
+    #[test]
+    fn hint_ghost_suppressed_under_reduce_motion() {
+        let (mut app, card) = ghost_app(true);
+        fire_hint(&mut app, card);
+
+        assert_eq!(
+            ghost_count(&mut app),
+            0,
+            "no ghost may spawn under reduce-motion; the static highlight is the whole story"
+        );
+    }
+
+    #[test]
+    fn state_change_despawns_the_ghost() {
+        let (mut app, card) = ghost_app(false);
+        fire_hint(&mut app, card);
+        assert_eq!(ghost_count(&mut app), 1);
+
+        app.world_mut().write_message(StateChangedEvent);
+        app.update();
+        app.update();
+
+        assert_eq!(
+            ghost_count(&mut app),
+            0,
+            "a board change makes the previewed move stale — the ghost must vanish"
+        );
+    }
+
+    #[test]
+    fn second_hint_replaces_the_first_ghost() {
+        let (mut app, card) = ghost_app(false);
+        fire_hint(&mut app, card.clone());
+        fire_hint(&mut app, card);
+        app.update();
+
+        assert_eq!(
+            ghost_count(&mut app),
+            1,
+            "re-hinting must replace the in-flight ghost, not stack a second"
+        );
+    }
 }