chore: delete dead code approved from the PR #166 sweep

Three items the multi-agent sweep flagged, now removed with user
approval (§8 for the solitaire_sync changes):

- WinCascadePlugin: never registered; handle_win_cascade in
  AnimationPlugin is the live win cascade and builds its own targets.
  Its now-orphaned helpers (win_scatter_targets, cascade_delay,
  WIN_CASCADE_INTERVAL_SECS) had no callers outside their own tests
  and go with it.
- SyncCompleteEvent: written by the pull-completion system, zero
  readers — UI reads SyncStatusResource instead.
- solitaire_sync::ApiError: unused by client and server; the merge_at
  crate-root re-export goes too (merge::merge_at stays for the merge
  module's own use).

ARCHITECTURE.md updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/sync_plugin.rs

@@ -22,12 +22,11 @@ use solitaire_data::{
     AchievementRecord, PlayerProgress, Replay, StatsSnapshot, SyncError, SyncProvider,
     save_achievements_to, save_progress_to, save_replay_history_to, save_stats_to,
 };
-use solitaire_sync::{SyncPayload, SyncResponse, merge};
+use solitaire_sync::{SyncPayload, merge};
 
 use crate::achievement_plugin::{AchievementsResource, AchievementsStoragePath};
 use crate::events::{
-    GameWonEvent, ManualSyncRequestEvent, SyncCompleteEvent, SyncConfigureRequestEvent,
-    WarningToastEvent,
+    GameWonEvent, ManualSyncRequestEvent, SyncConfigureRequestEvent, WarningToastEvent,
 };
 use crate::game_plugin::RecordingReplay;
 use crate::progress_plugin::{ProgressResource, ProgressStoragePath};
@@ -108,7 +107,6 @@ impl Plugin for SyncPlugin {
             .init_resource::<PullTask>()
             .init_resource::<PendingReplayUpload>()
             .add_message::<ManualSyncRequestEvent>()
-            .add_message::<SyncCompleteEvent>()
             .add_message::<SyncConfigureRequestEvent>()
             .add_message::<WarningToastEvent>();
 
@@ -198,7 +196,6 @@ fn poll_pull_result(
     achievements_path: Res<AchievementsStoragePath>,
     mut progress: ResMut<ProgressResource>,
     progress_path: Res<ProgressStoragePath>,
-    mut complete_writer: MessageWriter<SyncCompleteEvent>,
     mut configure_sync: MessageWriter<SyncConfigureRequestEvent>,
     mut warning_toast: MessageWriter<WarningToastEvent>,
 ) {
@@ -213,7 +210,7 @@ fn poll_pull_result(
     match result {
         Ok(remote) => {
             let local = build_payload(&stats.0, &achievements.0, &progress.0);
-            let (merged, conflicts) = merge(&local, &remote);
+            let (merged, _conflicts) = merge(&local, &remote);
 
             // Persist merged state atomically.
             if let Some(p) = &stats_path.0
@@ -233,17 +230,10 @@ fn poll_pull_result(
             }
 
             // Update in-world resources.
-            let now = Utc::now();
-            stats.0 = merged.stats.clone();
-            achievements.0 = merged.achievements.clone();
-            progress.0 = merged.progress.clone();
-            status.0 = SyncStatus::LastSynced(now);
-
-            complete_writer.write(SyncCompleteEvent(Ok(SyncResponse {
-                merged,
-                server_time: now,
-                conflicts,
-            })));
+            stats.0 = merged.stats;
+            achievements.0 = merged.achievements;
+            progress.0 = merged.progress;
+            status.0 = SyncStatus::LastSynced(Utc::now());
         }
         Err(SyncError::UnsupportedPlatform) => {
             // No backend configured — not an error, just leave status as Idle.
@@ -266,8 +256,7 @@ fn poll_pull_result(
             if matches!(e, SyncError::Auth(_)) {
                 configure_sync.write(SyncConfigureRequestEvent);
             }
-            status.0 = SyncStatus::Error(msg.clone());
-            complete_writer.write(SyncCompleteEvent(Err(msg)));
+            status.0 = SyncStatus::Error(msg);
         }
     }
 }