make turnout description more clear. fix curves from breaking running track.

src/engine/track.ts

@@ -406,6 +406,20 @@ export function allReachable(
 // Placement
 // ---------------------------------------------------------------------------
 
+/**
+ * Does this card carry the through route — an east-west road straight across it?
+ *
+ * The Running Track is the road trains run on, Limits to Limits, and every card standing in it has
+ * to pass traffic along the row. A curve does not: it has one road, from an east or west edge round
+ * to a 45° leg, and dropping one into the running row DEAD-ENDS the main there. Straights, turnouts,
+ * Limits signs, the Office and industries all carry it.
+ */
+export function carriesThroughTrack(card: TrackCard): boolean {
+  return connectionsFor(card).some(
+    ([a, b]) => (a === 'e' && b === 'w') || (a === 'w' && b === 'e'),
+  );
+}
+
 /**
  * Gap 4a — a placed card must connect to existing track, and "connect" means `joins`: ports meeting
  * AND, on a north or south edge, 45° legs on the same diagonal.
@@ -433,6 +447,10 @@ export function canPlaceAt(area: OfficeArea, coord: GridCoord, card: TrackCard):
   // with a Limits card stranded mid-track. The district below the Running Track is unbounded.
   if (coord.row === area.runningRow) {
     if (coord.col < area.limitsWest.col || coord.col > area.limitsEast.col) return false;
+    // And it must not BREAK the Running Track. A curve laid here has no east-west road, so the main
+    // stops dead at it — the bot did exactly this, turning the west end of its own Running Track
+    // into a stub and cutting the Office off from the Limits.
+    if (!carriesThroughTrack(card)) return false;
   }
 
   const ports: Port[] = ['n', 's', 'e', 'w'];