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/lib.rs

@@ -13,6 +13,7 @@ pub mod progress_plugin;
 pub mod resources;
 pub mod stats_plugin;
 pub mod table_plugin;
+pub mod time_attack_plugin;
 pub mod weekly_goals_plugin;
 
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource};
@@ -36,3 +37,6 @@ pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};
 pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
 pub use table_plugin::{PileMarker, TableBackground, TablePlugin};
+pub use time_attack_plugin::{
+    TimeAttackEndedEvent, TimeAttackPlugin, TimeAttackResource, TIME_ATTACK_DURATION_SECS,
+};