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/events.rs

@@ -5,7 +5,6 @@ use solitaire_core::KlondikePile;
 use solitaire_core::game_state::GameMode;
 use solitaire_core::{Card, Suit};
 use solitaire_data::AchievementRecord;
-use solitaire_sync::SyncResponse;
 
 /// Request to move `count` cards from `from` to `to`. Fired by input systems,
 /// consumed by `GamePlugin`.
@@ -248,16 +247,6 @@ pub struct ToggleLeaderboardRequestEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct ToggleHomeRequestEvent;
 
-/// Fired by `SyncPlugin` after a pull task resolves and the merged result has
-/// been persisted to disk. `Ok(SyncResponse)` carries the merged payload plus
-/// any `ConflictReport`s the merge produced. `Err(String)` carries a
-/// human-readable failure message (network, auth, serialization, etc.).
-///
-/// UI systems listen for this to refresh views without polling
-/// `SyncStatusResource`. See [ARCHITECTURE.md §4](../../ARCHITECTURE.md).
-#[derive(Message, Debug, Clone)]
-pub struct SyncCompleteEvent(pub Result<SyncResponse, String>);
-
 /// Generic informational toast message. Any system can fire this to display
 /// a short string to the player, e.g. "Locked — reach level 5".
 #[derive(Message, Debug, Clone)]