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/timing.rs

@@ -49,17 +49,6 @@ pub fn micro_vary(duration: f32, entity_index: u32) -> f32 {
     duration * (1.0 + variation)
 }
 
-/// Returns the pre-animation delay for card at `index` in a staggered cascade.
-///
-/// `delay = index × interval_secs`.
-#[inline]
-pub fn cascade_delay(index: usize, interval_secs: f32) -> f32 {
-    index as f32 * interval_secs
-}
-
-/// Recommended per-card interval for the win cascade (Normal speed).
-pub const WIN_CASCADE_INTERVAL_SECS: f32 = 0.018;
-
 /// Recommended per-card interval for deal animations (Normal speed).
 pub const DEAL_INTERVAL_SECS: f32 = 0.022;
 
@@ -138,21 +127,4 @@ mod tests {
         );
     }
 
-    #[test]
-    fn cascade_delay_zero_index_is_zero() {
-        assert_eq!(cascade_delay(0, 0.018), 0.0);
-    }
-
-    #[test]
-    fn cascade_delay_scales_linearly() {
-        let interval = 0.018;
-        for i in 0..52usize {
-            let expected = i as f32 * interval;
-            let actual = cascade_delay(i, interval);
-            assert!(
-                (actual - expected).abs() < 1e-6,
-                "cascade_delay({i}) = {actual}, expected {expected}"
-            );
-        }
-    }
 }