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/lib.rs

@@ -15,6 +15,7 @@ pub mod avatar_plugin;
 pub mod card_animation;
 pub mod card_plugin;
 pub mod challenge_plugin;
+pub mod cheat_sheet_plugin;
 pub mod contextual_tips_plugin;
 pub mod core_game_plugin;
 pub mod cursor_plugin;
@@ -27,6 +28,7 @@ pub mod font_plugin;
 pub mod game_plugin;
 pub mod help_plugin;
 pub mod home_plugin;
+pub mod hotkeys;
 pub mod hud_plugin;
 pub mod input_plugin;
 pub mod layout;
@@ -95,6 +97,7 @@ pub use card_plugin::{
 pub use challenge_plugin::{
     CHALLENGE_UNLOCK_LEVEL, ChallengeAdvancedEvent, ChallengePlugin, challenge_progress_label,
 };
+pub use cheat_sheet_plugin::{CheatSheetOverlay, CheatSheetPlugin};
 pub use contextual_tips_plugin::ContextualTipsPlugin;
 pub use core_game_plugin::CoreGamePlugin;
 pub use cursor_plugin::CursorPlugin;