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/lib.rs

@@ -83,9 +83,8 @@ pub use avatar_plugin::{AvatarFetchEvent, AvatarPlugin, AvatarResource};
 pub use card_animation::{
     AnimationChain, AnimationTuning, BufferedInput, CardAnimation, CardAnimationPlugin,
     DEAL_INTERVAL_SECS, DIAG_WINDOW_SIZE, FrameTimeDiagnostics, HoverState, InputBuffer,
-    InputPlatform, MAX_DURATION_SECS, MIN_DURATION_SECS, MotionCurve, WIN_CASCADE_INTERVAL_SECS,
-    WinCascadePlugin, cascade_delay, compute_duration, micro_vary, sample_curve,
-    win_scatter_targets,
+    InputPlatform, MAX_DURATION_SECS, MIN_DURATION_SECS, MotionCurve, compute_duration,
+    micro_vary, sample_curve,
 };
 pub use card_plugin::{
     CardEntity, CardImageSet, CardLabel, CardPlugin, HintHighlight, HintHighlightTimer,
@@ -107,7 +106,7 @@ pub use events::{
     HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent,
     NewGameRequestEvent, PauseRequestEvent, StartChallengeRequestEvent,
     StartDailyChallengeRequestEvent, StartDifficultyRequestEvent, StartPlayBySeedRequestEvent,
-    StartTimeAttackRequestEvent, StartZenRequestEvent, StateChangedEvent, SyncCompleteEvent,
+    StartTimeAttackRequestEvent, StartZenRequestEvent, StateChangedEvent,
     ToggleAchievementsRequestEvent, ToggleLeaderboardRequestEvent, ToggleProfileRequestEvent,
     ToggleSettingsRequestEvent, ToggleStatsRequestEvent, UndoRequestEvent, WinStreakMilestoneEvent,
     XpAwardedEvent,