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/core_game_plugin.rs

@@ -14,14 +14,14 @@ use crate::platform::{
 };
 use crate::{
     AchievementPlugin, AnimationPlugin, AssetSourcesPlugin, AutoCompletePlugin,
-    CardAnimationPlugin, CardPlugin, ChallengePlugin, ContextualTipsPlugin, CursorPlugin,
-    DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin, FontPlugin,
-    GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, OnboardingPlugin, PausePlugin,
-    PlayBySeedPlugin, ProfilePlugin, ProgressPlugin, RadialMenuPlugin, ReplayOverlayPlugin,
-    ReplayPlaybackPlugin, SafeAreaInsetsPlugin, SelectionPlugin, SettingsPlugin,
-    SolutionPlaybackPlugin, SplashPlugin, StatsPlugin, SyncProvider, TablePlugin, ThemePlugin,
-    ThemeRegistryPlugin, TimeAttackPlugin, TouchSelectionPlugin, UiFocusPlugin, UiModalPlugin,
-    UiTooltipPlugin, WeeklyGoalsPlugin, WhatsNewPlugin, WinSummaryPlugin,
+    CardAnimationPlugin, CardPlugin, ChallengePlugin, CheatSheetPlugin, ContextualTipsPlugin,
+    CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin,
+    FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, OnboardingPlugin,
+    PausePlugin, PlayBySeedPlugin, ProfilePlugin, ProgressPlugin, RadialMenuPlugin,
+    ReplayOverlayPlugin, ReplayPlaybackPlugin, SafeAreaInsetsPlugin, SelectionPlugin,
+    SettingsPlugin, SolutionPlaybackPlugin, SplashPlugin, StatsPlugin, SyncProvider, TablePlugin,
+    ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin, TouchSelectionPlugin, UiFocusPlugin,
+    UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WhatsNewPlugin, WinSummaryPlugin,
 };
 #[cfg(not(target_arch = "wasm32"))]
 use crate::{
@@ -117,6 +117,7 @@ impl Plugin for CoreGamePlugin {
             .add_plugins(OnboardingPlugin)
             .add_plugins(WhatsNewPlugin)
             .add_plugins(ContextualTipsPlugin)
+            .add_plugins(CheatSheetPlugin)
             .add_plugins(WinSummaryPlugin)
             .add_plugins(UiModalPlugin)
             .add_plugins(UiFocusPlugin)