Add Swap Home Bases — the audit's one escapee

Neutral, LOS to the other player, legal only when both home bases hold
an equal number of treasures. With this, every card in the 6th edition
game is implemented except Thumb Of God, which awaits its digital
redesign. 118 tests passing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/engine/test/expansion-combat.test.ts

@@ -209,3 +209,20 @@ describe("expansion combat cards", () => {
     }
   });
 });
+
+describe("swap home bases", () => {
+  it("trades homes when both bases hold equal treasures", () => {
+    let { state } = newGame();
+    const me = activePlayer(state);
+    const other = state.players.find((p) => p.id !== me.id)!;
+    other.position = { ...me.position }; // LOS guaranteed
+    const myHome = { ...me.home };
+    const theirHome = { ...other.home };
+    const shb = giveCard(state, me.id, "swap-home-bases");
+    state = must(state, me.id, {
+      type: "cast", instanceId: shb.instanceId, target: { kind: "player", playerId: other.id },
+    });
+    expect(cellKey(state.players.find((p) => p.id === me.id)!.home)).toBe(cellKey(theirHome));
+    expect(cellKey(state.players.find((p) => p.id === other.id)!.home)).toBe(cellKey(myHome));
+  });
+});