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

test/track.test.ts

@@ -7,6 +7,7 @@ import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 
 import type {
+  GameState,
   GridCoord,
   OfficeArea,
   RollingStock,
@@ -14,6 +15,8 @@ import type {
   TurnoutOrientation,
 } from '../src/engine/state.ts';
 import { coordKey } from '../src/engine/state.ts';
+import { createGame } from '../src/engine/setup.ts';
+import { applyIntent, areaOf } from '../src/engine/apply.ts';
 import type { MoveContext, Occupancy, Port } from '../src/engine/track.ts';
 import {
   allReachable,
@@ -95,6 +98,27 @@ const lockedFacility = (): TrackCard => ({
 
 const car = (): RollingStock => ({ type: 'boxcar', loaded: false });
 
+function straightCard(): TrackCard {
+  return {
+    geometry: { kind: 'track', geometry: 'straight', axis: 'ew' },
+    baseOperationalRail: true, standing: [], facility: null, modifiers: [], enhancements: [],
+  };
+}
+
+/** A minimal game wrapped around a prepared Office Area, for engine-level switching tests. */
+function gameWith(area: OfficeArea): GameState {
+  const s = createGame({
+    id: 'g', seed: 5,
+    config: {
+      mode: 'solitaire', victory: 'highestAfterDays', length: 'standard',
+      optionalRules: { reducedVisibility: false, sisterTrains: false, employeeRotation: false, emergencyToolbox: false },
+    },
+    playerNames: ['p'],
+  });
+  s.officeAreas.set(0, area);
+  return s;
+}
+
 function areaFrom(cards: Record<string, TrackCard>, officeCoord: GridCoord): OfficeArea {
   const grid = new Map<string, TrackCard>();
   for (const [k, v] of Object.entries(cards)) grid.set(k, v);
@@ -503,3 +527,43 @@ describe('curves are arcs, and arcs make sidings possible', () => {
     assert.ok(seen.has('0,1'), 'the siding does not rejoin the Running Track');
   });
 });
+
+describe('coupling lifts only the cars the crew ran over', () => {
+  it('leaves cars standing elsewhere in the district alone', () => {
+    // The reducer cleared EVERY card in the Office Area on any coupling, so picking up one boxcar
+    // deleted every car standing anywhere — including loads worked over several Stages onto an
+    // industry track the crew never went near.
+    const grid: Record<string, TrackCard> = {
+      '0,-1': straightCard(),
+      '0,0': straightCard(),
+      '0,1': straightCard(),
+      '0,2': straightCard(),
+    };
+    grid['0,-1']!.standing.push({ type: 'boxcar', loaded: false });   // on the path
+    grid['0,2']!.standing.push({ type: 'hopper', loaded: true });      // nowhere near it
+
+    const area = areaFrom(grid, { row: 0, col: 0 });
+    const s = gameWith(area);
+    s.trays.set('crew', {
+      id: 'crew', trainNumber: 1, trainIsExtra: false, engineFront: true, consist: [],
+      direction: 'west', facing: 'w', position: { at: 'grid', owner: 0, coord: { row: 0, col: 0 } }, movesUsed: 0,
+    });
+    s.clock.phase = 'localOps';
+    s.clock.currentActor = 0;
+    s.turn.option = 'switch';
+    s.turn.movesRemaining = 6;
+
+    const r = applyIntent(s, 0, { type: 'switch.move', trayId: 'crew', to: { row: 0, col: -1 }, reverse: false });
+    assert.ok(r.ok, 'the move should be legal');
+    assert.deepEqual(
+      s.trays.get('crew')!.consist.map((c: RollingStock) => c.type),
+      ['boxcar'],
+      'the crew should have picked up the car it ran over',
+    );
+    assert.equal(
+      areaOf(s, 0).grid.get('0,2')!.standing.length,
+      1,
+      'a car standing off the path was deleted by an unrelated coupling',
+    );
+  });
+});