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/io/save.js

@@ -4,10 +4,23 @@
 // a crashed browser does not take a career with it. A save is the whole game
 // state and nothing else — the engine keeps no state outside it, so this is a
 // complete record by construction.
+//
+// There is one slot per pack. A single global slot was fine while there was
+// one setting; the moment a second existed, switching settings would have
+// silently thrown away the other run — the guard in the app read the packId
+// off whatever was in the slot and started a new game when it did not match.
 
-const KEY = 'theladder.save.v1';
+const PREFIX = 'theladder.save.v1';
+// The slot as it was before packs were selectable: one save, no pack in the
+// key. Read once, for Corporate Ladder only, so a run in progress survives
+// this change. Never written.
+const LEGACY_KEY = PREFIX;
+const LEGACY_PACK = 'corporateladder';
+const LAST_PACK_KEY = 'theladder.pack.v1';
 const FORMAT = 1;
 
+const slotFor = (packId) => `${PREFIX}.${packId}`;
+
 /** Local storage throws in private windows and when site data is blocked. */
 function safely(action, fallback = null) {
   try {
@@ -17,24 +30,49 @@ function safely(action, fallback = null) {
   }
 }
 
-export function saveLocal(state) {
+export function saveLocal(packId, state) {
   return safely(() => {
-    localStorage.setItem(KEY, JSON.stringify({ format: FORMAT, savedAt: new Date().toISOString(), state }));
+    localStorage.setItem(
+      slotFor(packId),
+      JSON.stringify({ format: FORMAT, savedAt: new Date().toISOString(), state }),
+    );
     return true;
   }, false);
 }
 
-export function loadLocal() {
+/**
+ * The saved run for one pack, or null. Falls back to the pre-packs slot for
+ * Corporate Ladder, and only when that slot really holds a Corporate Ladder
+ * save — a stale slot from some other pack is not a reason to load a run into
+ * a game whose events do not match it.
+ */
+export function loadLocal(packId) {
   return safely(() => {
-    const raw = localStorage.getItem(KEY);
-    if (!raw) return null;
-    const parsed = JSON.parse(raw);
-    return parsed?.state ?? null;
+    const raw = localStorage.getItem(slotFor(packId));
+    if (raw) return JSON.parse(raw)?.state ?? null;
+    if (packId !== LEGACY_PACK) return null;
+    const legacy = localStorage.getItem(LEGACY_KEY);
+    if (!legacy) return null;
+    const state = JSON.parse(legacy)?.state ?? null;
+    return state?.meta?.packId === LEGACY_PACK ? state : null;
   });
 }
 
-export function clearLocal() {
-  return safely(() => { localStorage.removeItem(KEY); return true; }, false);
+export function clearLocal(packId) {
+  return safely(() => {
+    localStorage.removeItem(slotFor(packId));
+    if (packId === LEGACY_PACK) localStorage.removeItem(LEGACY_KEY);
+    return true;
+  }, false);
+}
+
+/** Which pack was last played, so a returning player lands back in it. */
+export function lastPlayed() {
+  return safely(() => localStorage.getItem(LAST_PACK_KEY));
+}
+
+export function rememberPack(packId) {
+  return safely(() => { localStorage.setItem(LAST_PACK_KEY, packId); return true; }, false);
 }
 
 /** True if the browser will actually remember anything. */