adding classification and division yard updates, train collisions, engine and car updates, turnout improvements.

src/engine/state.ts

@@ -202,8 +202,19 @@ export type CrewTray = {
   /** null while a local crew is switching without a train card. */
   trainNumber: number | null;
   trainIsExtra: boolean;
-  /** Which end the engine occupies (§A.3). */
-  engineFront: boolean;
+  /**
+   * WHERE THE ENGINE SITS IN THE TRAY, as an index into `consist`.
+   *
+   * A Crew Tray is an engine plus its Rolling Stock, and the engine may be PULLING (index 0, ahead
+   * of everything), PUSHING (index `consist.length`, behind everything) or somewhere in the middle
+   * doing both at once. That last case is why this is an index and not the boolean it replaced —
+   * `engineFront` was written in three places and read in none, so the engine had a position the
+   * game recorded and never used.
+   *
+   * The engine is NOT one of the `consist` entries: §8.2 counts the consist as Rolling Stock, and
+   * the four-car limit (§A.4) is a limit on cars, not on the locomotive hauling them.
+   */
+  engineAt: number;
   /** ORDERED, left-to-right. Max 4 including any caboose (§A.4). */
   consist: RollingStock[];
   direction: Direction;