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

example/todomvc/pocketbook/todomvc.cljs

@@ -1,6 +1,7 @@
 (ns pocketbook.todomvc
   "TodoMVC built on Pocketbook — offline-first, synced, Clojure-native."
   (:require [pocketbook.core :as pb]
+            [pocketbook.store.idb :as idb]
             [cljs.core.async :refer [go <!]]
             [clojure.string :as str]))
 
@@ -241,11 +242,11 @@
 
 (defn ^:export init []
   (go
-    (let [conn  (<! (pb/open "pocketbook-todomvc"))
-          todos (pb/synced-atom conn "todo"
+    (let [store (<! (idb/open "pocketbook-todomvc"))
+          todos (pb/synced-atom store "todo"
                   {:server "http://localhost:8090/sync"
                    :interval 15000})]
-      (reset! !conn conn)
+      (reset! !conn store)
       (reset! !todos todos)
       ;; Render + bind immediately (empty or stale is fine)
       (render!)