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/mod.rs

@@ -31,16 +31,6 @@
 //! ));
 //! ```
 //!
-//! # Win cascade with `Expressive` curve
-//!
-//! The existing `AnimationPlugin` drives the win cascade with `CardAnim`
-//! (linear). To use the curve-based cascade instead, disable
-//! `handle_win_cascade` in `AnimationPlugin` and register `WinCascadePlugin`
-//! (declared below) which uses `CardAnimation` + `MotionCurve::Expressive`.
-//!
-//! They **must not both be active** — both write to `Transform` on the same
-//! 52 entities and will race.
-//!
 //! # Coexistence rules
 //!
 //! | Condition | Safe? |
@@ -58,24 +48,21 @@ pub mod interaction;
 pub mod timing;
 pub mod tuning;
 
-pub use animation::{CardAnimation, win_scatter_targets};
+pub use animation::CardAnimation;
 pub use chain::AnimationChain;
 pub use curves::{MotionCurve, sample_curve};
 pub use diagnostics::{FrameTimeDiagnostics, WINDOW_SIZE as DIAG_WINDOW_SIZE};
 pub use interaction::{BufferedInput, HoverState, InputBuffer};
 pub use timing::{
-    DEAL_INTERVAL_SECS, MAX_DURATION_SECS, MIN_DURATION_SECS, WIN_CASCADE_INTERVAL_SECS,
-    cascade_delay, compute_duration, micro_vary,
+    DEAL_INTERVAL_SECS, MAX_DURATION_SECS, MIN_DURATION_SECS, compute_duration, micro_vary,
 };
 pub use tuning::{AnimationTuning, InputPlatform};
 
 use bevy::prelude::*;
 use bevy::window::RequestRedraw;
 
-use crate::card_plugin::CardEntity;
 use crate::events::{DrawRequestEvent, GameWonEvent, MoveRequestEvent, UndoRequestEvent};
 use crate::game_plugin::GameMutation;
-use crate::layout::LayoutResource;
 use crate::resources::DragState;
 
 use animation::advance_card_animations;
@@ -144,63 +131,6 @@ impl Plugin for CardAnimationPlugin {
     }
 }
 
-// ---------------------------------------------------------------------------
-// Optional: win cascade with Expressive curve
-// ---------------------------------------------------------------------------
-
-/// Optional plugin that replaces the linear win cascade in `AnimationPlugin`
-/// with an `Expressive`-curve cascade.
-///
-/// **Do not register this alongside `AnimationPlugin`'s win cascade** — they
-/// will race on the same card entities. To use this plugin, prevent
-/// `AnimationPlugin` from handling `GameWonEvent` (or remove it and manage
-/// win toasts manually).
-pub struct WinCascadePlugin;
-
-impl Plugin for WinCascadePlugin {
-    fn build(&self, app: &mut App) {
-        app.add_systems(Update, trigger_expressive_win_cascade.after(GameMutation));
-    }
-}
-
-/// Inserts `CardAnimation` (Expressive curve) on every card when `GameWonEvent` fires.
-///
-/// Cards scatter to 8 off-screen positions with per-card stagger. The z-lift
-/// creates a "burst" effect as cards fly outward.
-fn trigger_expressive_win_cascade(
-    mut events: MessageReader<GameWonEvent>,
-    cards: Query<(Entity, &Transform), With<CardEntity>>,
-    layout: Option<Res<LayoutResource>>,
-    mut commands: Commands,
-) {
-    if events.read().next().is_none() {
-        return;
-    }
-
-    let radius = layout.as_ref().map_or(800.0, |l| l.0.card_size.x * 8.0);
-
-    let targets = win_scatter_targets(radius);
-
-    for (index, (entity, transform)) in cards.iter().enumerate() {
-        let start_xy = transform.translation.truncate();
-        let start_z = transform.translation.z;
-        let target = targets[index % targets.len()];
-
-        commands.entity(entity).insert(
-            CardAnimation::slide(
-                start_xy,
-                start_z,
-                target,
-                start_z + 60.0,
-                MotionCurve::Expressive,
-            )
-            .with_delay(cascade_delay(index, WIN_CASCADE_INTERVAL_SECS))
-            .with_duration(0.65)
-            .with_z_lift(25.0),
-        );
-    }
-}
-
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -433,19 +363,4 @@ mod tests {
         let state = HoverState::default();
         assert!(state.entity.is_none());
     }
-
-    #[test]
-    fn win_scatter_produces_eight_distinct_points() {
-        let targets = win_scatter_targets(600.0);
-        assert_eq!(targets.len(), 8);
-        // All must be different.
-        for i in 0..8 {
-            for j in (i + 1)..8 {
-                assert_ne!(
-                    targets[i], targets[j],
-                    "scatter targets {i} and {j} must be distinct"
-                );
-            }
-        }
-    }
 }