refactor: extract shared .cljc library with store protocol

Move core, sync, and transit from platform-specific .clj/.cljs to
shared .cljc files with reader conditionals. This enables testing
the full sync logic on the JVM and using SyncedAtom from Clojure
clients.

Key changes:
- PStore protocol (store.cljc) decouples core from storage backend
- IDB store (store/idb.cljs) and memory store (store/memory.cljc)
- SyncedAtom implements CLJ IDeref/IAtom/IRef + CLJS equivalents
- Sync client uses java.net.http on CLJ, fetch on CLJS
- SSE remains CLJS-only; JVM clients use polling
- API change: store passed explicitly instead of pb/open
- 7 new JVM tests: local ops, persistence, watches, two-client sync
- 28 tests total, 87 assertions, all passing

test/pocketbook/server_test.clj

@@ -41,19 +41,19 @@
                 (.header "Accept" "application/transit+json")
                 (.GET)
                 (.build))
-        resp (.send client req (HttpResponse$BodyHandlers/ofByteArray))]
+        resp (.send client req (HttpResponse$BodyHandlers/ofString))]
     {:status (.statusCode resp)
      :body   (t/decode (.body resp))}))
 
 (defn- post-transit [path body]
-  (let [bytes (t/encode body)
-        req   (-> (HttpRequest/newBuilder)
-                  (.uri (URI. (url path)))
-                  (.header "Content-Type" "application/transit+json")
-                  (.header "Accept" "application/transit+json")
-                  (.POST (HttpRequest$BodyPublishers/ofByteArray bytes))
-                  (.build))
-        resp  (.send client req (HttpResponse$BodyHandlers/ofByteArray))]
+  (let [encoded (t/encode body)
+        req     (-> (HttpRequest/newBuilder)
+                    (.uri (URI. (url path)))
+                    (.header "Content-Type" "application/transit+json")
+                    (.header "Accept" "application/transit+json")
+                    (.POST (HttpRequest$BodyPublishers/ofString encoded))
+                    (.build))
+        resp    (.send client req (HttpResponse$BodyHandlers/ofString))]
     {:status (.statusCode resp)
      :body   (t/decode (.body resp))}))