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/transit_test.clj

@@ -30,8 +30,8 @@
                :version 5}]]
     (is (= data (t/decode (t/encode data))))))
 
-(deftest encode-str-roundtrip
+(deftest encode-returns-string
   (let [v {:hello "world" :nums [1 2 3]}
-        s (t/encode-str v)]
+        s (t/encode v)]
     (is (string? s))
     (is (= v (t/decode s)))))