Expand game engine, replay, tests, and documentation

src/engine/intents.ts

@@ -33,6 +33,8 @@ export type Intent =
   // -- New Train Phase (§7)
   | { type: 'newTrain.placeCar'; trayId: TrayId; carType: CarType; loaded: boolean }
   | { type: 'newTrain.passCar'; trayId: TrayId }
+  /** Q9 — run a second, identical section behind a train that is due out this Stage. */
+  | { type: 'newTrain.secondSection'; trainNumber: number }
   // -- Mainline Phase (§8.1) — the Superintendent's clearance ruling
   | { type: 'mainline.clearance'; allow: boolean }
   | { type: 'redFlag.play' }
@@ -80,6 +82,7 @@ export type RejectionCode =
   | 'NO_PENDING_DECISION'
   | 'NOT_UPGRADEABLE'
   | 'FACILITY_LOCKED'
+  | 'NOT_IMPLEMENTED'
   | 'NO_TRAIN_AT_OFFICE';
 
 export type Rejection = { code: RejectionCode; message: string };