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/help_plugin.rs

@@ -53,7 +53,7 @@ fn spawn_help_screen(commands: &mut Commands) {
         "-- Overlays --".to_string(),
         "  S            Toggle stats / progression".to_string(),
         "  H or ?       Toggle this help".to_string(),
-        "  Esc          Pause (placeholder)".to_string(),
+        "  Esc          Pause / resume".to_string(),
         String::new(),
         "Press H or ? to close".to_string(),
     ];