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/index.js

@@ -0,0 +1,23 @@
+// The packs this build ships with.
+//
+// A setting is a content pack, and this is the only list of them. Adding one
+// is an import and an entry — no engine change, no interface change, and
+// nothing anywhere else that names it. `packs[0]` is what a first-time player
+// starts in.
+//
+// The bundler follows static imports from the entry module, so appearing in
+// this file is also what gets a pack into `dist/theladder.html`.
+
+import { corporateLadder } from './corporateladder/index.js';
+import { frontier } from './frontier/index.js';
+
+export const packs = [corporateLadder, frontier];
+
+/** A pack by id, or undefined. Used to resume into whatever was last played. */
+export function packById(id) {
+  return packs.find((pack) => pack.id === id);
+}
+
+export const defaultPack = packs[0];
+
+export default packs;