Support agreed extended turns in Basic and Advanced

game/victory-calendar.ts

@@ -0,0 +1,35 @@
+/** Provisional calendar for an agreement to play another full turn.
+ * Older games omit the marker and retain the printed ten-turn calendar. */
+export type VictoryCalendar = {
+  turn: number;
+  victoryFinalTurn?: number;
+};
+
+export class VictoryCalendarError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'VictoryCalendarError';
+  }
+}
+
+/** Return the agreed final turn while rejecting a game that has already
+ * advanced beyond its calendar. Does not mutate or migrate old games. */
+export function victoryFinalTurn(calendar: VictoryCalendar): number {
+  const turn = calendar?.turn;
+  const finalTurn = calendar?.victoryFinalTurn === undefined ? 10 : calendar.victoryFinalTurn;
+  if (!Number.isSafeInteger(turn) || turn < 1 ||
+      !Number.isSafeInteger(finalTurn) || finalTurn < 10 || turn > finalTurn)
+    throw new VictoryCalendarError('The current turn must belong to the agreed victory calendar.');
+  return finalTurn;
+}
+
+/** Call after unanimous acceptance, before the turn advances. An agreement
+ * earlier than the current final turn keeps that final turn unchanged. At the
+ * final turn it moves the fallback exactly once to the next full turn. */
+export function continueVictoryCalendar(calendar: VictoryCalendar): number {
+  const finalTurn = victoryFinalTurn(calendar);
+  if (calendar.turn !== finalTurn) return finalTurn;
+  if (!Number.isSafeInteger(finalTurn + 1))
+    throw new VictoryCalendarError('The agreed final turn cannot be extended safely.');
+  return finalTurn + 1;
+}