Add The Frontier, a second setting, and a picker to choose between them

Cold Fork is a freight yard: 29 events across two stages, a four-day cycle, and
its own relationship and skill vocabulary. src/engine/ was not touched. Adding it
needed three changes outside the engine: one save slot per pack, app.js holding
the current pack, and turn.js no longer naming the mailroom (DECISIONS #34).
build.js now handles imports wrapped across lines. A conformance suite runs over
every registered pack. DECISIONS #35 records three calls on open questions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RxdJFbLq1rBKsUeticGV1g

content/frontier/events/index.js

@@ -0,0 +1,8 @@
+import { routine } from './routine.js';
+import { people } from './people.js';
+import { promotion } from './promotion.js';
+import { deputy } from './deputy.js';
+import { hardTimes } from './hard-times.js';
+
+export const events = [...routine, ...people, ...promotion, ...deputy, ...hardTimes];
+export default events;