Show the whole Division mainline. district now has option for auto hiding on phases where not needed. scaffold for multi-player geometry showing division. regions on mainline cards implemented. bug fix on laying rail cards. optimized replay (carried links forward). Add sounds, first pass

src/engine/state.ts

@@ -228,7 +228,24 @@ export type CrewTray = {
  * A train part-way across a Mainline card. Crossing time is measured in STAGES (Q1/Q2), so the
  * regions the placeholder used are gone — the cells printed on the cards are decoration.
  */
-export type Transit = { tray: TrayId; stagesRemaining: number; direction: Direction };
+/**
+ * A train crossing a Mainline card.
+ *
+ * `stagesTotal` is what the crossing cost when the train ENTERED, and it exists so the Division map
+ * can say where on the card a train is. §2.1 divides a Mainline card into two regions and §8.2 has
+ * a train move one region per Stage; the engine models crossing as a countdown of Stages instead,
+ * so position has to be derived — and it cannot be recomputed later, because a modifier played onto
+ * the card mid-crossing would change the answer and make the train appear to jump backwards.
+ *
+ * Read by nothing that decides anything: no legality check, no movement, no bot. It records what
+ * already happened so the picture can be drawn.
+ */
+export type Transit = {
+  tray: TrayId;
+  stagesRemaining: number;
+  stagesTotal: number;
+  direction: Direction;
+};
 
 export type DivisionNode =
   | { kind: 'divisionPoint'; side: Direction; holding: TrayId[] }