chore: durable state snippet w/interface guard

 - regions for snippets
 - more concise

publicFacet was a poor example of a baggage key

snippets/zoe/src/02-state.js

@@ -1,13 +1,17 @@
 import { Far } from '@endo/far';
 
 // #region startfn
+// #region heap-state
 export const start = () => {
   let value = 0;
+  // #endregion heap-state
   const get = () => value;
   const set = v => (value = v);
 
+  // #region fresh-export
   return {
     publicFacet: Far('ValueCell', { get, set }),
   };
+  // #endregion fresh-export
 };
 // #endregion startfn

snippets/zoe/src/02b-state-durable.js

@@ -0,0 +1,57 @@
+// @ts-check
+// #region contract
+import { M } from '@endo/patterns';
+// #region import-zone
+import { makeDurableZone } from '@agoric/zone/durable.js';
+// #endregion import-zone
+
+// #region interface-guard
+const RoomI = M.interface('Room', {
+  getId: M.call().returns(M.number()),
+  incr: M.call().returns(M.number()),
+  decr: M.call().returns(M.number()),
+});
+
+const RoomMakerI = M.interface('RoomMaker', {
+  makeRoom: M.call().returns(M.remotable()),
+});
+// #endregion interface-guard
+
+// #region export-prepare
+export const prepare = (_zcf, _privateArgs, baggage) => {
+  // #endregion export-prepare
+  // #region zone1
+  const zone = makeDurableZone(baggage);
+  const rooms = zone.mapStore('rooms');
+  // #endregion zone1
+
+  // #region exoclass
+  const makeRoom = zone.exoClass('Room', RoomI, (id) => ({ id, count: 0 }), {
+    getId() {
+      return this.state.id;
+    },
+    incr() {
+      this.state.count += 1;
+      return this.state.count;
+    },
+    decr() {
+      this.state.count -= 1;
+      return this.state.count;
+    },
+  });
+  // #endregion exoclass
+
+  // #region exo
+  const publicFacet = zone.exo('RoomMaker', RoomMakerI, {
+    makeRoom() {
+      const room = makeRoom();
+      const id = rooms.size;
+      rooms.init(id, room);
+      return room;
+    },
+  });
+
+  return { publicFacet };
+  // #endregion exo
+};
+// #endregion contract