feat(engine): hold-/ hotkey cheat sheet + unified binding table (Phase J)

Phase J audit findings: the visible focus ring the doc asked for
already exists (FocusOverlay singleton, 2px accent ring, breathing
pulse, reduce-motion aware — landed after the doc was written), and
post-Phase-C every modal already opens and dismisses keyboard-only.
The real gaps were two silently diverging static hotkey tables
(onboarding's slide vs Help's list — onboarding still said 'Mode
Launcher (then 1-5)') and no way to see the bindings mid-game.

- New crate::hotkeys module owns THE binding table (21 rows, verified
  against a grep inventory of every just_pressed(KeyCode::..) handler);
  rows carry an `essential` flag — the onboarding slide teaches that
  subset, the cheat sheet shows everything.
- New cheat_sheet_plugin: hold `/` for a right-anchored reference
  overlay of every binding; release hides it. Deliberately not a
  spawn_modal modal (momentary reference, closer to a tooltip), never
  spawns while a modal owns the screen (also keeps it out of the
  seed-entry field), inert on touch builds.
- Onboarding's stale local table deleted in favour of the shared one
  (copy updated: Home naming, hold-to-repeat undo, added H).

5 new tests (table integrity, essential-subset size, uniqueness,
show/hide driver, modal suppression). Workspace + clippy green.

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

solitaire_engine/src/cheat_sheet_plugin.rs

@@ -0,0 +1,194 @@
+//! Hold-`/` hotkey cheat sheet (Phase J).
+//!
+//! While `/` is held on a keyboard platform, a lightweight overlay
+//! lists every binding from [`crate::hotkeys::HOTKEYS`] — the same
+//! table the onboarding slide teaches from, so the two can never
+//! disagree. Releasing the key hides it instantly; it never captures
+//! input, never pauses the game, and never spawns while a modal owns
+//! the screen (which also keeps it out of the seed-entry text field's
+//! way).
+//!
+//! Not a `spawn_modal` modal on purpose: modals are sticky and guarded;
+//! this is a momentary reference card, closer to a tooltip than a
+//! dialog.
+
+use bevy::input::ButtonInput;
+use bevy::prelude::*;
+
+use crate::font_plugin::FontResource;
+use crate::hotkeys::HOTKEYS;
+use crate::platform::SHOW_KEYBOARD_ACCELERATORS;
+use crate::ui_modal::ModalScrim;
+use crate::ui_theme::{
+    ACCENT_PRIMARY, BG_ELEVATED, BORDER_STRONG, HighContrastBorder, RADIUS_MD, TEXT_PRIMARY,
+    TEXT_SECONDARY, TYPE_BODY, TYPE_CAPTION, VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3, VAL_SPACE_4,
+    Z_TOOLTIP,
+};
+
+/// Marker on the cheat-sheet overlay root.
+#[derive(Component, Debug)]
+pub struct CheatSheetOverlay;
+
+/// Registers the hold-`/` driver. Inert on touch-first builds.
+pub struct CheatSheetPlugin;
+
+impl Plugin for CheatSheetPlugin {
+    fn build(&self, app: &mut App) {
+        app.init_resource::<ButtonInput<KeyCode>>()
+            .add_systems(Update, drive_cheat_sheet);
+    }
+}
+
+/// Shows the overlay while `/` is held (and no modal owns the screen);
+/// hides it the frame the key releases.
+fn drive_cheat_sheet(
+    keys: Res<ButtonInput<KeyCode>>,
+    scrims: Query<(), With<ModalScrim>>,
+    existing: Query<Entity, With<CheatSheetOverlay>>,
+    font_res: Option<Res<FontResource>>,
+    mut commands: Commands,
+) {
+    if !SHOW_KEYBOARD_ACCELERATORS {
+        return;
+    }
+    let held = keys.pressed(KeyCode::Slash);
+    if held && existing.is_empty() && scrims.is_empty() {
+        spawn_cheat_sheet(&mut commands, font_res.as_deref());
+    } else if !held {
+        for entity in &existing {
+            commands.entity(entity).despawn();
+        }
+    }
+}
+
+fn spawn_cheat_sheet(commands: &mut Commands, font_res: Option<&FontResource>) {
+    let font_handle = font_res.map(|f| f.0.clone()).unwrap_or_default();
+    let font_header = TextFont {
+        font: font_handle.clone(),
+        font_size: TYPE_BODY,
+        ..default()
+    };
+    let font_keys = TextFont {
+        font: font_handle.clone(),
+        font_size: TYPE_CAPTION,
+        ..default()
+    };
+    let font_desc = TextFont {
+        font: font_handle,
+        font_size: TYPE_CAPTION,
+        ..default()
+    };
+
+    commands
+        .spawn((
+            CheatSheetOverlay,
+            Node {
+                position_type: PositionType::Absolute,
+                right: Val::Px(16.0),
+                top: Val::Percent(8.0),
+                flex_direction: FlexDirection::Column,
+                row_gap: VAL_SPACE_1,
+                padding: UiRect::all(VAL_SPACE_4),
+                border: UiRect::all(Val::Px(1.0)),
+                border_radius: BorderRadius::all(Val::Px(RADIUS_MD)),
+                max_height: Val::Percent(84.0),
+                overflow: Overflow::scroll_y(),
+                ..default()
+            },
+            BackgroundColor(BG_ELEVATED),
+            BorderColor::all(BORDER_STRONG),
+            HighContrastBorder::with_default(BORDER_STRONG),
+            GlobalZIndex(Z_TOOLTIP),
+        ))
+        .with_children(|panel| {
+            panel.spawn((
+                Text::new("Keyboard shortcuts"),
+                font_header.clone(),
+                TextColor(TEXT_PRIMARY),
+                Node {
+                    margin: UiRect::bottom(VAL_SPACE_2),
+                    ..default()
+                },
+            ));
+            for row in HOTKEYS {
+                panel
+                    .spawn(Node {
+                        flex_direction: FlexDirection::Row,
+                        column_gap: VAL_SPACE_3,
+                        ..default()
+                    })
+                    .with_children(|line| {
+                        line.spawn((
+                            Text::new(row.keys),
+                            font_keys.clone(),
+                            TextColor(ACCENT_PRIMARY),
+                            Node {
+                                min_width: Val::Px(110.0),
+                                ..default()
+                            },
+                        ));
+                        line.spawn((
+                            Text::new(row.description),
+                            font_desc.clone(),
+                            TextColor(TEXT_SECONDARY),
+                        ));
+                    });
+            }
+        });
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn app() -> App {
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins)
+            .add_plugins(CheatSheetPlugin);
+        app.update();
+        app
+    }
+
+    fn overlay_count(app: &mut App) -> usize {
+        app.world_mut()
+            .query::<&CheatSheetOverlay>()
+            .iter(app.world())
+            .count()
+    }
+
+    #[test]
+    fn holding_slash_shows_and_release_hides() {
+        let mut app = app();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::Slash);
+        app.update();
+        assert_eq!(overlay_count(&mut app), 1, "held / must show the sheet");
+        // Still held on later frames: exactly one overlay, no stacking.
+        app.update();
+        assert_eq!(overlay_count(&mut app), 1);
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .release(KeyCode::Slash);
+        app.update();
+        app.update();
+        assert_eq!(overlay_count(&mut app), 0, "release must hide the sheet");
+    }
+
+    #[test]
+    fn suppressed_while_a_modal_is_open() {
+        let mut app = app();
+        app.world_mut().spawn(ModalScrim);
+        app.update();
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::Slash);
+        app.update();
+        assert_eq!(
+            overlay_count(&mut app),
+            0,
+            "the sheet must not spawn over a modal (or into a text field)"
+        );
+    }
+}