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

src/ui/screens/turn.js

@@ -37,7 +37,7 @@ export function turnScreen(pack, state, schema, view, actions) {
           : choosingView(pack, view, actions),
       ),
       h('aside', { class: 'people' },
-        h('h2', { class: 'people__heading' }, 'The mailroom'),
+        h('h2', { class: 'people__heading' }, pack.peopleHeading ?? 'The people'),
         (pack.characters ?? []).map((character) => characterCard(pack, state, schema, character)),
       ),
     ),
@@ -50,6 +50,7 @@ export function turnScreen(pack, state, schema, view, actions) {
       h('button', { class: 'button button--quiet', onClick: actions.onExportFeedback },
         'Export feedback log'),
       h('button', { class: 'button button--quiet', onClick: actions.onNewGame }, 'New game'),
+      h('button', { class: 'button button--quiet', onClick: actions.onPacks }, 'Change setting'),
     ),
   );
 }