Add three-member short alliance prototype

game/alliance-victory-option.ts

@@ -4,9 +4,9 @@ import {
   type TwoPlayerControllers,
 } from './two-player-controllers';
 
-/** GF9's optional two-member alliance / three-stronghold variant. The
- * two-person four-stronghold rule still applies, as do ordinary solo targets.
- * Callers add any separately agreed increase after this base target. */
+/** GF9's shorter alliance target, also used by the staged three-member and
+ * unlimited variants. The two-person four-stronghold rule still applies, as
+ * do ordinary solo targets. Callers add any agreed increase afterward. */
 export function allianceVictoryTarget(
   seats: readonly string[],
   controllers: TwoPlayerControllers | null | undefined,