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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "theladder",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "private": true,
   "description": "A turn-based career simulation game. Engine is theme-agnostic; settings are content packs.",
   "type": "module",