feat(engine): InfoToastEvent — show locked-mode messages on-screen

Replaces silent info!() log calls with on-screen toasts when the player
presses Z/X/T without reaching the required unlock level. Any system
can now fire InfoToastEvent(message) to surface a brief text overlay
without depending on a specific plugin.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -75,3 +75,8 @@ pub struct ManualSyncRequestEvent;
 /// confirmation window sends `NewGameRequestEvent`.
 #[derive(Event, Debug, Clone, Copy, Default)]
 pub struct NewGameConfirmEvent;
+
+/// Generic informational toast message. Any system can fire this to display
+/// a short string to the player, e.g. "Locked — reach level 5".
+#[derive(Event, Debug, Clone)]
+pub struct InfoToastEvent(pub String);