feat(engine): MoveRejectedEvent + PausePlugin (Esc)

- New MoveRejectedEvent fires from end_drag when the cursor is over
  a real pile but the placement is illegal. AudioPlugin plays
  card_invalid.wav on it.
- New PausePlugin + PausedResource: Esc toggles a full-window
  overlay and the flag. tick_elapsed_time and advance_time_attack
  skip work while paused. Help cheat sheet updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -34,6 +34,16 @@ pub struct NewGameRequestEvent {
 #[derive(Event, Debug, Clone, Copy, Default)]
 pub struct StateChangedEvent;
 
+/// Fired by input/UI systems when a player attempts to drop dragged cards
+/// on a real pile but the move violates the rules. Drives the
+/// `card_invalid.wav` SFX. Not fired for drops in empty space.
+#[derive(Event, Debug, Clone)]
+pub struct MoveRejectedEvent {
+    pub from: PileType,
+    pub to: PileType,
+    pub count: usize,
+}
+
 /// Fired once when the active game transitions to won.
 #[derive(Event, Debug, Clone, Copy)]
 pub struct GameWonEvent {