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/events.rs

@@ -4,6 +4,7 @@ use bevy::prelude::Message;
 use solitaire_core::game_state::GameMode;
 use solitaire_core::pile::PileType;
 use solitaire_data::AchievementRecord;
+use solitaire_sync::SyncResponse;
 
 /// Request to move `count` cards from `from` to `to`. Fired by input systems,
 /// consumed by `GamePlugin`.
@@ -78,6 +79,16 @@ pub struct AchievementUnlockedEvent(pub AchievementRecord);
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct ManualSyncRequestEvent;
 
+/// Fired by `SyncPlugin` after a pull task resolves and the merged result has
+/// been persisted to disk. `Ok(SyncResponse)` carries the merged payload plus
+/// any `ConflictReport`s the merge produced. `Err(String)` carries a
+/// human-readable failure message (network, auth, serialization, etc.).
+///
+/// UI systems listen for this to refresh views without polling
+/// `SyncStatusResource`. See [ARCHITECTURE.md §4](../../ARCHITECTURE.md).
+#[derive(Message, Debug, Clone)]
+pub struct SyncCompleteEvent(pub Result<SyncResponse, String>);
+
 /// Fired by `InputPlugin` when N is pressed while a game is in progress
 /// but confirmation has not yet been received. The animation plugin shows
 /// a "Press N again to confirm" toast. A second N press within the