feat(engine): emit SyncCompleteEvent on pull resolve

ARCHITECTURE.md §5 lists SyncCompleteEvent(Result<SyncResponse, String>) as
a cross-system event, but it was never declared or fired. Add the message
to events.rs, register it in SyncPlugin, and emit it from poll_pull_result
on both the success path (carrying the merged payload + conflicts as
SyncResponse) and the failure path (carrying the user-facing error
message). UI/persistence systems can now react to sync completion without
polling SyncStatusResource.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -69,7 +69,7 @@ pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
     ForfeitEvent, GameWonEvent, HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent,
     MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent, NewGameRequestEvent,
-    StateChangedEvent, UndoRequestEvent, XpAwardedEvent,
+    StateChangedEvent, SyncCompleteEvent, UndoRequestEvent, XpAwardedEvent,
 };
 pub use game_plugin::{ConfirmNewGameScreen, GameMutation, GameOverScreen, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};