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_sync/src/lib.rs

@@ -13,7 +13,7 @@ pub mod stats;
 pub mod theme_store;
 
 pub use achievements::AchievementRecord;
-pub use merge::{merge, merge_at};
+pub use merge::merge;
 pub use progress::{PlayerProgress, level_for_xp};
 pub use stats::StatsSnapshot;
 pub use theme_store::{ThemeCatalogEntry, ThemeCatalogResponse};
@@ -98,30 +98,3 @@ pub struct LeaderboardEntry {
     /// When this entry was last recorded.
     pub recorded_at: DateTime<Utc>,
 }
-
-// ---------------------------------------------------------------------------
-// Error types
-// ---------------------------------------------------------------------------
-
-/// Errors returned by the sync server in `application/json` error bodies.
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, thiserror::Error)]
-pub enum ApiError {
-    /// The request could not be authenticated (missing or invalid JWT).
-    #[error("unauthorized")]
-    Unauthorized,
-    /// The supplied credentials were incorrect.
-    #[error("invalid credentials")]
-    InvalidCredentials,
-    /// A username that was requested for registration is already taken.
-    #[error("username already taken")]
-    UsernameTaken,
-    /// The request payload was too large (> 1 MB).
-    #[error("payload too large")]
-    PayloadTooLarge,
-    /// The request body could not be parsed.
-    #[error("bad request: {0}")]
-    BadRequest(String),
-    /// An unexpected server-side error occurred.
-    #[error("internal server error")]
-    Internal,
-}