Implement Homeworld mechanics through Caladan reinforcement checkpoint

Connect typed Homeworld custody, transport, battle, income, revival and Caladan reinforcement flows with private multiplayer recovery and controls. Preserve the optimized test workflow and keep unfinished rules modes gated. Verified with 3682 offline tests, typecheck, lint, production build and 40 HTTP tests; existing saved games remain unchanged.

game/junction-offer.ts

@@ -0,0 +1,49 @@
+import type { Game } from './engine';
+import { homeworldContext } from './homeworld-game';
+import { homeworldPopulations } from './homeworld-population';
+import { HomeworldCustodyError } from './homeworld-custody';
+
+export type JunctionOffer = {
+  event: string;
+  turn: number;
+  owner: string;
+  recipient: string;
+  rate: 'half' | 'full';
+};
+
+/** A public, optional quote for the recipient's current shipment opportunity.
+ * It is not a second action allowance or a durable grant for a later turn. */
+export function currentJunctionOffer(g: Game): JunctionOffer | null {
+  const offer = g.junctionOffer;
+  if (!offer || !junctionSponsor(g) || offer.owner !== junctionSponsor(g) ||
+      offer.turn !== g.turn || offer.recipient !== g.active || g.phase !== 5 ||
+      g.status !== 'playing' || g.players.find((p) => p.id === offer.recipient)?.shipped)
+    return null;
+  return offer;
+}
+
+export function junctionSponsor(g: Game): string | null {
+  if (!g.homeworlds?.custody) return null;
+  return homeworldPopulations(homeworldContext(g), g.homeworlds.custody)
+    .find((world) => world.card === 'junction' && world.side === 'high')?.native ?? null;
+}
+
+/** Only public turn and population data bind the opportunity. Hand changes or
+ * the sponsor's private balance cannot reveal themselves through this token. */
+export function junctionOfferEvent(g: Game): string {
+  return JSON.stringify([g.turn, g.phase, g.active, junctionSponsor(g),
+    g.players.find((p) => p.id === g.active)?.shipped ?? null]);
+}
+
+export function junctionOfferIntegrity(g: Game): void {
+  const offer = g.junctionOffer;
+  if (offer == null) return;
+  if (typeof offer !== 'object' || Array.isArray(offer) ||
+      Object.keys(offer).sort().join(',') !== 'event,owner,rate,recipient,turn' ||
+      typeof offer.event !== 'string' || !offer.event.length ||
+      !Number.isSafeInteger(offer.turn) || offer.turn < 1 || offer.turn > g.turn ||
+      !g.homeworlds?.custody || !g.players.some((p) => p.id === offer.owner && p.faction === 'guild') ||
+      offer.owner === offer.recipient || !g.players.some((p) => p.id === offer.recipient) ||
+      !['half', 'full'].includes(offer.rate))
+    throw new HomeworldCustodyError('The saved Junction offer needs its original Guild, recipient, turn and tariff.');
+}