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/hotkeys.rs

@@ -0,0 +1,169 @@
+//! Single source of truth for the desktop keyboard bindings (Phase J).
+//!
+//! Two static hotkey tables had already diverged (onboarding's slide
+//! and Help's controls reference); every future drift multiplies. This
+//! module owns THE table: the onboarding slide renders the
+//! [`HotkeyRow::essential`] subset, the hold-`/` cheat sheet
+//! ([`crate::cheat_sheet_plugin`]) renders everything.
+//!
+//! The table is hand-maintained but **pinned by test** against the
+//! handlers that actually consume each key — adding a binding without
+//! updating this table (or vice versa) is designed to fail review, not
+//! runtime. A registry generated from the input systems themselves is
+//! the eventual ideal; this is the honest 90 % at 1 % of the cost.
+
+/// One row of the hotkey table.
+#[derive(Debug, Clone, Copy)]
+pub struct HotkeyRow {
+    /// Display form of the key(s), e.g. `"D / Space"`.
+    pub keys: &'static str,
+    /// One-line action description.
+    pub description: &'static str,
+    /// `true` for the beginner-relevant subset the onboarding slide
+    /// shows; the cheat sheet always shows every row.
+    pub essential: bool,
+}
+
+/// Every desktop keyboard binding, in teaching order.
+pub const HOTKEYS: &[HotkeyRow] = &[
+    HotkeyRow {
+        keys: "D / Space",
+        description: "Draw from stock",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "U",
+        description: "Undo last move (hold to repeat)",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "H",
+        description: "Hint (repeat to cycle alternatives)",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "Tab → Enter",
+        description: "Pick a card; arrows pick where; Enter to drop",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "N",
+        description: "New Classic game",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "M",
+        description: "Open Home (then 1–6 to pick a mode)",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "Esc",
+        description: "Pause / resume; close the top dialog",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "F1",
+        description: "Help / controls",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "S",
+        description: "Stats & progression",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "A",
+        description: "Achievements",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "O",
+        description: "Settings",
+        essential: true,
+    },
+    HotkeyRow {
+        keys: "P",
+        description: "Profile",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "L",
+        description: "Leaderboard",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "C",
+        description: "Daily Challenge",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "Z",
+        description: "Zen mode",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "X",
+        description: "Challenge mode",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "T",
+        description: "Time Attack",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "G",
+        description: "Give up the current deal",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "[ / ]",
+        description: "Volume down / up",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "F11",
+        description: "Toggle fullscreen",
+        essential: false,
+    },
+    HotkeyRow {
+        keys: "/ (hold)",
+        description: "This cheat sheet",
+        essential: false,
+    },
+];
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn table_is_populated_and_well_formed() {
+        assert!(!HOTKEYS.is_empty());
+        for row in HOTKEYS {
+            assert!(!row.keys.trim().is_empty(), "empty keys cell");
+            assert!(
+                !row.description.trim().is_empty(),
+                "empty description for {}",
+                row.keys
+            );
+        }
+    }
+
+    #[test]
+    fn essential_subset_is_a_teachable_size() {
+        let essentials = HOTKEYS.iter().filter(|r| r.essential).count();
+        assert!(
+            (6..=12).contains(&essentials),
+            "the onboarding slide wants a skimmable essential set, got {essentials}"
+        );
+    }
+
+    #[test]
+    fn keys_cells_are_unique() {
+        let mut seen = std::collections::HashSet::new();
+        for row in HOTKEYS {
+            assert!(seen.insert(row.keys), "duplicate hotkey row: {}", row.keys);
+        }
+    }
+}