feat(engine): add HelpPlugin (H/?) and Challenge cleared toast

- HelpPlugin: full-window cheat sheet listing every keybinding,
  toggled with H or ?. Three unit tests cover open/close/slash.
- AnimationPlugin: ChallengeAdvancedEvent now surfaces as a
  3-second "Challenge N cleared!" toast.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/help_plugin.rs

@@ -0,0 +1,163 @@
+//! Toggleable on-screen help / cheat sheet showing keyboard bindings.
+//!
+//! Press **H** (or `?`) to toggle. Listed shortcuts are grouped by intent —
+//! gameplay, modes, and overlays.
+
+use bevy::prelude::*;
+
+/// Marker on the help overlay root node.
+#[derive(Component, Debug)]
+pub struct HelpScreen;
+
+pub struct HelpPlugin;
+
+impl Plugin for HelpPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(Update, toggle_help_screen);
+    }
+}
+
+fn toggle_help_screen(
+    mut commands: Commands,
+    keys: Res<ButtonInput<KeyCode>>,
+    screens: Query<Entity, With<HelpScreen>>,
+) {
+    let pressed_help = keys.just_pressed(KeyCode::KeyH) || keys.just_pressed(KeyCode::Slash);
+    if !pressed_help {
+        return;
+    }
+    if let Ok(entity) = screens.get_single() {
+        commands.entity(entity).despawn_recursive();
+    } else {
+        spawn_help_screen(&mut commands);
+    }
+}
+
+fn spawn_help_screen(commands: &mut Commands) {
+    let lines: Vec<String> = vec![
+        "=== Controls ===".to_string(),
+        String::new(),
+        "-- Gameplay --".to_string(),
+        "  D            Draw from stock".to_string(),
+        "  U            Undo last move".to_string(),
+        "  Drag         Move cards between piles".to_string(),
+        "  Click stock  Draw".to_string(),
+        String::new(),
+        "-- New Game --".to_string(),
+        "  N            New Classic game".to_string(),
+        "  C            Start today's daily challenge".to_string(),
+        "  Z            Start a Zen game (level 5+)".to_string(),
+        "  X            Start the next Challenge (level 5+)".to_string(),
+        "  T            Start a Time Attack session (level 5+)".to_string(),
+        String::new(),
+        "-- Overlays --".to_string(),
+        "  S            Toggle stats / progression".to_string(),
+        "  H or ?       Toggle this help".to_string(),
+        "  Esc          Pause (placeholder)".to_string(),
+        String::new(),
+        "Press H or ? to close".to_string(),
+    ];
+
+    commands
+        .spawn((
+            HelpScreen,
+            Node {
+                position_type: PositionType::Absolute,
+                left: Val::Percent(0.0),
+                top: Val::Percent(0.0),
+                width: Val::Percent(100.0),
+                height: Val::Percent(100.0),
+                flex_direction: FlexDirection::Column,
+                justify_content: JustifyContent::Center,
+                align_items: AlignItems::Center,
+                row_gap: Val::Px(4.0),
+                ..default()
+            },
+            BackgroundColor(Color::srgba(0.0, 0.0, 0.0, 0.88)),
+            ZIndex(210),
+        ))
+        .with_children(|b| {
+            for line in lines {
+                b.spawn((
+                    Text::new(line),
+                    TextFont {
+                        font_size: 22.0,
+                        ..default()
+                    },
+                    TextColor(Color::srgb(0.95, 0.95, 0.90)),
+                ));
+            }
+        });
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn headless_app() -> App {
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins).add_plugins(HelpPlugin);
+        app.init_resource::<ButtonInput<KeyCode>>();
+        app.update();
+        app
+    }
+
+    #[test]
+    fn pressing_h_spawns_help_screen() {
+        let mut app = headless_app();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::KeyH);
+        app.update();
+
+        assert_eq!(
+            app.world_mut()
+                .query::<&HelpScreen>()
+                .iter(app.world())
+                .count(),
+            1
+        );
+    }
+
+    #[test]
+    fn pressing_h_twice_closes_help_screen() {
+        let mut app = headless_app();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::KeyH);
+        app.update();
+
+        {
+            let mut input = app.world_mut().resource_mut::<ButtonInput<KeyCode>>();
+            input.release(KeyCode::KeyH);
+            input.clear();
+            input.press(KeyCode::KeyH);
+        }
+        app.update();
+
+        assert_eq!(
+            app.world_mut()
+                .query::<&HelpScreen>()
+                .iter(app.world())
+                .count(),
+            0
+        );
+    }
+
+    #[test]
+    fn pressing_slash_also_toggles_help() {
+        let mut app = headless_app();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::Slash);
+        app.update();
+
+        assert_eq!(
+            app.world_mut()
+                .query::<&HelpScreen>()
+                .iter(app.world())
+                .count(),
+            1
+        );
+    }
+}