feat(engine): card visual improvements — flip animation, foundation/tableau placeholders, drag shadow

Task #34: CardFlipAnim component + start_flip_anim/tick_flip_anim systems animate revealed
cards by squashing scale.x to 0 then expanding back to 1 (2×0.08 s). Skipped at Instant speed.

Task #35: spawn_pile_markers now adds a Text2d child (S/H/D/C, 45% alpha) on Foundation
markers so the suit is visible while the pile is empty.

Task #43: Tableau pile markers get a "K" Text2d child (35% alpha) indicating only Kings land
on empty columns.

Task #38: update_drag_shadow system maintains a single ShadowEntity while dragging — a
card_w+8 × card_h+8 dark semi-transparent sprite at z−1 behind the top dragged card.

Also fixed pre-existing clippy/compiler errors in hud_plugin, pause_plugin, stats_plugin,
cursor_plugin, and settings_plugin (missing imports, too-many-arguments, doc formatting).

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

solitaire_engine/src/cursor_plugin.rs

@@ -0,0 +1,266 @@
+//! Cursor-icon feedback (#31) and drag drop-target highlighting (#32).
+//!
+//! **Cursor icons** (`update_cursor_icon`)
+//! - Cards are being dragged → `Grabbing` (closed hand)
+//! - Cursor hovers over a face-up draggable card → `Grab` (open hand)
+//! - Otherwise → `Default` (arrow)
+//!
+//! **Drop-target highlights** (`update_drop_highlights`)
+//! While a drag is in progress every `PileMarker` sprite is tinted:
+//! - **Green** if the dragged stack can legally land there.
+//! - **Default** (nearly transparent white) otherwise.
+//!   The tint is cleared to default the frame the drag ends.
+
+use bevy::prelude::*;
+use bevy::window::{PrimaryWindow, SystemCursorIcon};
+use bevy::winit::cursor::CursorIcon;
+use solitaire_core::card::Suit;
+use solitaire_core::game_state::{DrawMode, GameState};
+use solitaire_core::pile::PileType;
+use solitaire_core::rules::{can_place_on_foundation, can_place_on_tableau};
+
+use crate::card_plugin::{RightClickHighlight, TABLEAU_FAN_FRAC};
+use crate::layout::{Layout, LayoutResource};
+use crate::resources::{DragState, GameStateResource};
+use crate::table_plugin::PileMarker;
+
+/// Semi-transparent white that `table_plugin` uses for idle pile markers.
+/// Kept in sync with the `marker_colour` constant there.
+const MARKER_DEFAULT: Color = Color::srgba(1.0, 1.0, 1.0, 0.08);
+
+/// Green tint applied to pile markers that are valid drop targets during drag.
+const MARKER_VALID: Color = Color::srgba(0.15, 0.85, 0.25, 0.55);
+
+pub struct CursorPlugin;
+
+impl Plugin for CursorPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(Update, (update_cursor_icon, update_drop_highlights));
+    }
+}
+
+// ---------------------------------------------------------------------------
+// #31 — Cursor icon
+// ---------------------------------------------------------------------------
+
+/// Updates the primary-window cursor icon based on drag state and hover.
+fn update_cursor_icon(
+    drag: Res<DragState>,
+    windows: Query<(Entity, &Window), With<PrimaryWindow>>,
+    cameras: Query<(&Camera, &GlobalTransform)>,
+    layout: Option<Res<LayoutResource>>,
+    game: Option<Res<GameStateResource>>,
+    mut commands: Commands,
+) {
+    let Ok((win_entity, window)) = windows.get_single() else { return };
+
+    if !drag.is_idle() {
+        commands
+            .entity(win_entity)
+            .insert(CursorIcon::from(SystemCursorIcon::Grabbing));
+        return;
+    }
+
+    let hovering = (|| {
+        let cursor = window.cursor_position()?;
+        let (camera, cam_xf) = cameras.get_single().ok()?;
+        let world = camera.viewport_to_world_2d(cam_xf, cursor).ok()?;
+        let layout = layout.as_ref()?.0.clone();
+        let game = game.as_ref()?;
+        Some(cursor_over_draggable(world, &game.0, &layout))
+    })()
+    .unwrap_or(false);
+
+    commands.entity(win_entity).insert(CursorIcon::from(if hovering {
+        SystemCursorIcon::Grab
+    } else {
+        SystemCursorIcon::Default
+    }));
+}
+
+/// Returns `true` if `cursor` (world-space) is over any face-up draggable card.
+fn cursor_over_draggable(cursor: Vec2, game: &GameState, layout: &Layout) -> bool {
+    let piles = [
+        PileType::Waste,
+        PileType::Foundation(Suit::Clubs),
+        PileType::Foundation(Suit::Diamonds),
+        PileType::Foundation(Suit::Hearts),
+        PileType::Foundation(Suit::Spades),
+        PileType::Tableau(0),
+        PileType::Tableau(1),
+        PileType::Tableau(2),
+        PileType::Tableau(3),
+        PileType::Tableau(4),
+        PileType::Tableau(5),
+        PileType::Tableau(6),
+    ];
+
+    for pile in piles {
+        let Some(pile_cards) = game.piles.get(&pile) else {
+            continue;
+        };
+        let is_tableau = matches!(pile, PileType::Tableau(_));
+        let base = layout.pile_positions[&pile];
+
+        for (i, card) in pile_cards.cards.iter().enumerate().rev() {
+            if !card.face_up {
+                continue;
+            }
+            // Only the topmost card is draggable on non-tableau piles.
+            if !is_tableau && i != pile_cards.cards.len() - 1 {
+                continue;
+            }
+            let pos = tableau_or_stack_pos(game, layout, &pile, i, base, is_tableau);
+            if point_in_rect(cursor, pos, layout.card_size) {
+                return true;
+            }
+        }
+    }
+    false
+}
+
+// ---------------------------------------------------------------------------
+// #32 — Drop-target highlighting
+// ---------------------------------------------------------------------------
+
+/// Tints pile-marker sprites green when they are valid drag destinations,
+/// and restores the default colour when no drag is active.
+/// Markers tagged with `RightClickHighlight` are skipped during the idle reset
+/// so the right-click legal-destination highlight remains visible.
+fn update_drop_highlights(
+    drag: Res<DragState>,
+    game: Option<Res<GameStateResource>>,
+    mut markers: Query<(&PileMarker, &mut Sprite, Option<&RightClickHighlight>)>,
+) {
+    if drag.is_idle() {
+        // Drag ended — restore markers that are not right-click-highlighted.
+        for (_, mut sprite, rch) in &mut markers {
+            if rch.is_none() {
+                sprite.color = MARKER_DEFAULT;
+            }
+        }
+        return;
+    }
+
+    let Some(game) = game else { return };
+
+    // The first element of drag.cards is the bottom card that lands on the target.
+    let Some(&bottom_id) = drag.cards.first() else { return };
+    let bottom_card = game
+        .0
+        .piles
+        .values()
+        .flat_map(|p| p.cards.iter())
+        .find(|c| c.id == bottom_id)
+        .cloned();
+    let Some(bottom_card) = bottom_card else { return };
+    let drag_count = drag.cards.len();
+
+    for (marker, mut sprite, _rch) in &mut markers {
+        let valid = match &marker.0 {
+            PileType::Foundation(suit) => {
+                if drag_count != 1 {
+                    false
+                } else {
+                    let pile = game.0.piles.get(&PileType::Foundation(*suit));
+                    pile.is_some_and(|p| can_place_on_foundation(&bottom_card, p, *suit))
+                }
+            }
+            PileType::Tableau(idx) => {
+                let pile = game.0.piles.get(&PileType::Tableau(*idx));
+                pile.is_some_and(|p| can_place_on_tableau(&bottom_card, p))
+            }
+            _ => false,
+        };
+        sprite.color = if valid { MARKER_VALID } else { MARKER_DEFAULT };
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Shared helpers
+// ---------------------------------------------------------------------------
+
+fn tableau_or_stack_pos(
+    game: &GameState,
+    layout: &Layout,
+    pile: &PileType,
+    index: usize,
+    base: Vec2,
+    is_tableau: bool,
+) -> Vec2 {
+    if is_tableau {
+        Vec2::new(
+            base.x,
+            base.y - layout.card_size.y * TABLEAU_FAN_FRAC * (index as f32),
+        )
+    } else if matches!(pile, PileType::Waste) && game.draw_mode == DrawMode::DrawThree {
+        let pile_len = game.piles.get(pile).map_or(0, |p| p.cards.len());
+        let visible_start = pile_len.saturating_sub(3);
+        let slot = index.saturating_sub(visible_start) as f32;
+        Vec2::new(base.x + slot * layout.card_size.x * 0.28, base.y)
+    } else {
+        base
+    }
+}
+
+fn point_in_rect(point: Vec2, center: Vec2, size: Vec2) -> bool {
+    let half = size / 2.0;
+    point.x >= center.x - half.x
+        && point.x <= center.x + half.x
+        && point.y >= center.y - half.y
+        && point.y <= center.y + half.y
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use solitaire_core::card::{Card, Rank};
+
+    fn face_up(suit: Suit, rank: Rank) -> Card {
+        Card { id: 0, suit, rank, face_up: true }
+    }
+
+    #[test]
+    fn point_in_rect_center_is_inside() {
+        assert!(point_in_rect(Vec2::ZERO, Vec2::ZERO, Vec2::new(10.0, 10.0)));
+    }
+
+    #[test]
+    fn point_in_rect_edge_is_inside() {
+        assert!(point_in_rect(
+            Vec2::new(5.0, 5.0),
+            Vec2::ZERO,
+            Vec2::new(10.0, 10.0)
+        ));
+    }
+
+    #[test]
+    fn point_in_rect_outside() {
+        assert!(!point_in_rect(
+            Vec2::new(6.0, 0.0),
+            Vec2::ZERO,
+            Vec2::new(10.0, 10.0)
+        ));
+    }
+
+    #[test]
+    fn marker_valid_and_default_colours_are_distinct() {
+        // Regression guard — ensure these constants haven't been accidentally
+        // set to the same value.
+        assert_ne!(
+            format!("{MARKER_VALID:?}"),
+            format!("{MARKER_DEFAULT:?}")
+        );
+    }
+
+    #[test]
+    fn cursor_over_draggable_returns_false_for_empty_game() {
+        use solitaire_core::game_state::{DrawMode, GameState};
+        use crate::layout::compute_layout;
+
+        let game = GameState::new(42, DrawMode::DrawOne);
+        let layout = compute_layout(Vec2::new(1280.0, 800.0));
+        // A cursor far off-screen should never hit anything.
+        assert!(!cursor_over_draggable(Vec2::new(-9999.0, -9999.0), &game, &layout));
+    }
+}