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>

ARCHITECTURE.md

@@ -145,7 +145,6 @@ Shared API contract types imported by both the game client (`solitaire_data`) an
 Owns:
 - `SyncPayload`, `SyncResponse`, `ConflictReport`
 - `ChallengeGoal`, `LeaderboardEntry`
-- `ApiError` enum
 - Merge logic (pure functions, no I/O)
 
 ### `solitaire_data`
@@ -257,7 +256,7 @@ solitaire_sync::merge(local, remote)
     │
     ▼
 Write merged result to disk
-    │  fires SyncCompleteEvent
+    │
     ▼
 Bevy main thread reads updated StatsResource
 ```
@@ -377,7 +376,6 @@ struct StateChangedEvent;
 struct CardFlippedEvent(u32);
 struct GameWonEvent { score: i32, time_seconds: u64 }
 struct AchievementUnlockedEvent(AchievementRecord);
-struct SyncCompleteEvent(Result<SyncResponse, String>);
 ```
 
 ### Layout System