Implement nose coupling so run-arounds work, and stop coupling wiping the district

src/engine/apply.ts

@@ -689,7 +689,26 @@ function execute(s: GameState, player: PlayerIndex, i: Intent): GameEvent[] {
         },
       ];
       if (dest.couples.length > 0) {
-        events.push({ type: 'carsCoupled', trayId: i.trayId, at: i.to, stock: dest.couples });
+        // Name the cards the cars came from. The reducer used to clear every card in the district
+        // instead, so one coupling anywhere deleted every standing car and every industry track in
+        // the Office Area — loads worked over several Stages vanished when a crew picked up a single
+        // boxcar somewhere else entirely.
+        const lifted = [
+          ...dest.path.map((step) => step.coord),
+          i.to,
+        ].filter((c) => carsOn(areaOf(s, player).grid.get(coordKey(c)) ?? emptyCard()).length > 0);
+        // §A.3 — "engines also have couplers on the front end, so a train can pick cars up onto
+        // its nose". Running forward the engine meets cars head-on and takes them in front; backing
+        // up, they couple behind. Which end they land on is the whole point of a run-around: it
+        // decides which car is next to come off.
+        events.push({
+          type: 'carsCoupled',
+          trayId: i.trayId,
+          at: i.to,
+          stock: dest.couples,
+          from: lifted,
+          toNose: !i.reverse,
+        });
       }
       return events;
     }
@@ -992,10 +1011,14 @@ export function reduce(s: GameState, e: GameEvent): void {
     case 'carsCoupled': {
       const tray = s.trays.get(e.trayId)!;
       const area = areaOf(s, tray.position.at === 'grid' ? tray.position.owner : 0);
-      tray.consist.push(...e.stock);
-      // Every card along the path is cleared; the event carries what was taken.
-      for (const card of area.grid.values()) {
-        if (card.standing.length > 0) card.standing = [];
+      if (e.toNose) tray.consist.unshift(...e.stock);
+      else tray.consist.push(...e.stock);
+      // ONLY the cards the crew ran over. Clearing the whole grid emptied industry tracks the crew
+      // never went near.
+      for (const coord of e.from) {
+        const card = area.grid.get(coordKey(coord));
+        if (!card) continue;
+        card.standing = [];
         if (card.facility) card.facility.industryTrack.cars = [];
       }
       break;
@@ -1485,6 +1508,18 @@ export function checkEnhancementPlacement(
   }
 }
 
+/** A stand-in with nothing on it, so a missing card reads as "no cars here" rather than throwing. */
+function emptyCard(): TrackCard {
+  return {
+    geometry: { kind: 'limits' },
+    baseOperationalRail: false,
+    standing: [],
+    facility: null,
+    modifiers: [],
+    enhancements: [],
+  };
+}
+
 /** Removes a played card from its owner's hand and sends it to the Salvage Yard. */
 function spendCard(s: GameState, player: PlayerIndex, cardId: CardId): void {
   s.decks.hands.set(player, (s.decks.hands.get(player) ?? []).filter((c) => c !== cardId));