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/card_animation/animation.rs

@@ -201,27 +201,6 @@ pub(crate) fn advance_card_animations(
     }
 }
 
-// ---------------------------------------------------------------------------
-// Win cascade
-// ---------------------------------------------------------------------------
-
-/// Win-cascade scatter targets — 8 points beyond the window edges.
-///
-/// Scaled by `radius` (pass `layout.card_size.x * 8.0` for a good result).
-pub fn win_scatter_targets(radius: f32) -> [Vec2; 8] {
-    let r = radius;
-    [
-        Vec2::new(r, r),
-        Vec2::new(-r, r),
-        Vec2::new(r, -r),
-        Vec2::new(-r, -r),
-        Vec2::new(0.0, r),
-        Vec2::new(0.0, -r),
-        Vec2::new(r, 0.0),
-        Vec2::new(-r, 0.0),
-    ]
-}
-
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -305,20 +284,4 @@ mod tests {
         assert!((anim.z_lift - 12.0).abs() < 1e-6);
     }
 
-    #[test]
-    fn win_scatter_has_eight_targets() {
-        let targets = win_scatter_targets(800.0);
-        assert_eq!(targets.len(), 8);
-    }
-
-    #[test]
-    fn win_scatter_targets_are_off_center() {
-        for t in win_scatter_targets(400.0) {
-            let dist = t.length();
-            assert!(
-                dist > 100.0,
-                "scatter target should be well off-center: {t:?}"
-            );
-        }
-    }
 }