feat(engine,core): add Time Attack mode + unlocks panel

- Core: GameMode::TimeAttack variant (no scoring/undo changes — session marker only)
- Engine: TimeAttackPlugin with TimeAttackResource, TimeAttackEndedEvent,
  T hotkey (gated to level >= 5), auto-deal on win, summary toast
- Engine: Stats overlay (S) gains an Unlocks subsection (card backs /
  backgrounds, sorted/deduped) and a live Time Attack panel while active

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/animation_plugin.rs

@@ -12,6 +12,7 @@ use crate::events::{AchievementUnlockedEvent, GameWonEvent};
 use crate::game_plugin::GameMutation;
 use crate::layout::LayoutResource;
 use crate::progress_plugin::LevelUpEvent;
+use crate::time_attack_plugin::TimeAttackEndedEvent;
 use crate::weekly_goals_plugin::WeeklyGoalCompletedEvent;
 
 /// Duration of a card slide (move) animation in seconds.
@@ -22,6 +23,7 @@ const ACHIEVEMENT_TOAST_SECS: f32 = 3.0;
 const LEVELUP_TOAST_SECS: f32 = 3.0;
 const DAILY_TOAST_SECS: f32 = 3.0;
 const WEEKLY_TOAST_SECS: f32 = 3.0;
+const TIME_ATTACK_TOAST_SECS: f32 = 5.0;
 const CASCADE_STAGGER: f32 = 0.05;
 const CASCADE_DURATION: f32 = 0.5;
 
@@ -59,6 +61,7 @@ impl Plugin for AnimationPlugin {
             .add_event::<LevelUpEvent>()
             .add_event::<DailyChallengeCompletedEvent>()
             .add_event::<WeeklyGoalCompletedEvent>()
+            .add_event::<TimeAttackEndedEvent>()
             .add_systems(
                 Update,
                 (
@@ -68,6 +71,7 @@ impl Plugin for AnimationPlugin {
                     handle_levelup_toast,
                     handle_daily_toast,
                     handle_weekly_toast,
+                    handle_time_attack_toast,
                     tick_toasts,
                 )
                     .after(GameMutation),
@@ -181,6 +185,19 @@ fn handle_weekly_toast(
     }
 }
 
+fn handle_time_attack_toast(
+    mut commands: Commands,
+    mut events: EventReader<TimeAttackEndedEvent>,
+) {
+    for ev in events.read() {
+        spawn_toast(
+            &mut commands,
+            format!("Time Attack: {} win{}", ev.wins, if ev.wins == 1 { "" } else { "s" }),
+            TIME_ATTACK_TOAST_SECS,
+        );
+    }
+}
+
 fn tick_toasts(
     mut commands: Commands,
     time: Res<Time>,