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

deps.edn

@@ -1,5 +1,6 @@
 {:paths ["src"]
  :deps  {org.clojure/clojure          {:mvn/version "1.12.0"}
+         org.clojure/core.async        {:mvn/version "1.7.701"}
          http-kit/http-kit             {:mvn/version "2.8.0"}
          com.cognitect/transit-clj     {:mvn/version "1.0.333"}
          com.taoensso/nippy            {:mvn/version "3.4.2"}