fix(multi): resolve 26 bugs found in comprehensive codebase review

Core fixes (issues #12, #13, #22):
- #12: undo now preserves score delta instead of restoring snapshot score
- #13: take_from_foundation defaults to false (non-standard house rule)
- #22: check_win validates full suit sequence, not just card count

Engine fixes:
- #8:  replay keyboard input guard against non-replay state
- #9:  help modal scrims.is_empty() guard added
- #10: settings modal scrims.is_empty() guard added
- #11: sync_plugin builds payload at poll time (not task-spawn time)
- #14: server replay mode case-sensitivity fix ("Classic")
- #15: play_by_seed_plugin confirmed flag set to true on launch
- #16: replay back-step debounce via Local<bool> + StateChangedEvent;
       register StateChangedEvent in ReplayOverlayPlugin (fixes 52 tests)
- #17: time-attack timer ignores win-summary overlay
- #18: HUD dropdown glyphs U+25BE → U+2193 (FiraMono-safe arrow)
- #19: theme plugin applies immediate visual update on A→B→A switch
- #20: SyncAuthError / SyncBusyOverlay split into separate entities so
       auth errors are visible after busy overlay is hidden
- #21: handle_forfeit ordered before update_stats_on_new_game
- #23: server merge uses correct avg_time_seconds and games_lost math
- #24: win_summary migrated to ModalScrim pattern
- #25: card_animation apply_deferred between animation systems
- #26: cursor_plugin HashMap access uses .get() with fallback
- #27: auto_complete mid-sequence deactivation guard
- #28: feedback_anim SettleAnim ordered before FoundationFlourish
- #29: achievement_plugin iterates all win events; adds scrims guard
- #30: leaderboard modal scrims.is_empty() guard added
- #31: server auth tmp file cleanup on rename failure
- #32: sync_setup modal scrims.is_empty() guard added
- #33: font_plugin uses match fallback; TokioRuntimeResource graceful
       current-thread fallback on runtime init failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/cursor_plugin.rs

@@ -382,8 +382,8 @@ fn update_drop_target_overlays(
 /// for everything else it is card-sized. Replicated here rather than
 /// imported because `pile_drop_rect` is private to `input_plugin` and
 /// this overlay is the only other consumer.
-fn drop_overlay_rect(pile: &PileType, layout: &Layout, game: &GameState) -> (Vec2, Vec2) {
-    let centre = layout.pile_positions[pile];
+fn drop_overlay_rect(pile: &PileType, layout: &Layout, game: &GameState) -> Option<(Vec2, Vec2)> {
+    let centre = layout.pile_positions.get(pile).copied()?;
     if matches!(pile, PileType::Tableau(_)) {
         let card_count = game.piles.get(pile).map_or(0, |p| p.cards.len());
         if card_count > 1 {
@@ -393,13 +393,13 @@ fn drop_overlay_rect(pile: &PileType, layout: &Layout, game: &GameState) -> (Vec
             let bottom_edge = bottom_card_centre_y - layout.card_size.y / 2.0;
             let span_height = top_edge - bottom_edge;
             let new_centre_y = (top_edge + bottom_edge) / 2.0;
-            return (
+            return Some((
                 Vec2::new(centre.x, new_centre_y),
                 Vec2::new(layout.card_size.x, span_height),
-            );
+            ));
         }
     }
-    (centre, layout.card_size)
+    Some((centre, layout.card_size))
 }
 
 /// Spawns one overlay parent (fill) plus four edge sprites (outline) at
@@ -410,7 +410,10 @@ fn spawn_drop_target_overlay(
     layout: &Layout,
     game: &GameState,
 ) {
-    let (centre, size) = drop_overlay_rect(pile, layout, game);
+    let Some((centre, size)) = drop_overlay_rect(pile, layout, game) else {
+        warn!("drop_overlay_rect: pile {pile:?} not in layout, skipping overlay");
+        return;
+    };
     let edge = DROP_TARGET_OUTLINE_PX;
 
     commands