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

bb.edn

@@ -14,7 +14,7 @@
 
   test {:doc "Run all server tests"
         :task (let [expr (str "(require 'pocketbook.db-test 'pocketbook.transit-test"
-                              " 'pocketbook.server-test)"
+                              " 'pocketbook.server-test 'pocketbook.core-test)"
                               " (let [r (clojure.test/run-all-tests #\"pocketbook\\..*\")]"
                               " (System/exit (if (and (zero? (:fail r)) (zero? (:error r))) 0 1)))")]
                 (shell "clj" "-M:dev" "-e" expr))}