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/onboarding_plugin.rs

@@ -84,66 +84,11 @@ struct OnboardingSkipButton;
 pub struct OnboardingSlideIndex(pub u8);
 
 // ---------------------------------------------------------------------------
-// Slide data — hotkey rows are taken verbatim from `help_plugin.rs` so the
-// two screens stay in sync without a shared abstraction.
+// Slide data — the hotkey slide renders the essential subset of the
+// shared table in `crate::hotkeys` (Phase J unified the previously
+// diverging copies here and in help_plugin).
 // ---------------------------------------------------------------------------
 
-/// A single `key — description` pair shown on slide 3.
-#[cfg(not(target_os = "android"))]
-struct HotkeyRow {
-    keys: &'static str,
-    description: &'static str,
-}
-
-/// Most-used shortcuts from the `help_plugin` canonical list.
-///
-/// Updating the list in `help_plugin.rs` should be mirrored here. The
-/// ARCHITECTURE.md decision log calls out that we copy values rather than
-/// refactor the help plugin.
-#[cfg(not(target_os = "android"))]
-const HOTKEYS: &[HotkeyRow] = &[
-    HotkeyRow {
-        keys: "D / Space",
-        description: "Draw from stock",
-    },
-    HotkeyRow {
-        keys: "U",
-        description: "Undo last move",
-    },
-    HotkeyRow {
-        keys: "Tab → Enter",
-        description: "Pick a card; arrows pick where; Enter to drop",
-    },
-    HotkeyRow {
-        keys: "N",
-        description: "New Classic game",
-    },
-    HotkeyRow {
-        keys: "M",
-        description: "Open Mode Launcher (then 1–5 to pick)",
-    },
-    HotkeyRow {
-        keys: "S",
-        description: "Stats & progression",
-    },
-    HotkeyRow {
-        keys: "A",
-        description: "Achievements",
-    },
-    HotkeyRow {
-        keys: "O",
-        description: "Settings",
-    },
-    HotkeyRow {
-        keys: "Esc",
-        description: "Pause / resume",
-    },
-    HotkeyRow {
-        keys: "F1",
-        description: "Help / controls",
-    },
-];
-
 // ---------------------------------------------------------------------------
 // Plugin
 // ---------------------------------------------------------------------------
@@ -446,8 +391,10 @@ fn spawn_slide_hotkeys(commands: &mut Commands, font_res: Option<&FontResource>)
     spawn_modal(commands, OnboardingScreen, Z_ONBOARDING, |card| {
         spawn_modal_header(card, "Keyboard shortcuts", font_res);
 
-        // Vertical list of `key — description` rows, same chip style as HelpScreen.
-        for row in HOTKEYS {
+        // Vertical list of `key — description` rows, same chip style as
+        // HelpScreen. Essential subset only — the full table lives on
+        // the hold-`/` cheat sheet.
+        for row in crate::hotkeys::HOTKEYS.iter().filter(|r| r.essential) {
             card.spawn(Node {
                 flex_direction: FlexDirection::Row,
                 align_items: AlignItems::Center,
@@ -826,22 +773,15 @@ mod tests {
     }
 
     // -----------------------------------------------------------------------
-    // Hotkey list is non-empty (guards against accidental truncation)
+    // Hotkey slide renders a non-empty essential subset (the table's own
+    // integrity tests live in `crate::hotkeys`)
     // -----------------------------------------------------------------------
 
     #[test]
-    fn hotkey_list_is_non_empty() {
-        assert!(!HOTKEYS.is_empty(), "HOTKEYS must not be empty");
-    }
-
-    #[test]
-    fn all_hotkey_rows_have_non_empty_fields() {
-        for row in HOTKEYS {
-            assert!(!row.keys.is_empty(), "hotkey key field must not be empty");
-            assert!(
-                !row.description.is_empty(),
-                "hotkey description must not be empty"
-            );
-        }
+    fn hotkey_slide_subset_is_non_empty() {
+        assert!(
+            crate::hotkeys::HOTKEYS.iter().any(|r| r.essential),
+            "the onboarding slide needs at least one essential hotkey"
+        );
     }
 }