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

src/pocketbook/store.cljc

@@ -0,0 +1,23 @@
+(ns pocketbook.store
+  "Storage protocol for Pocketbook.
+   All methods return core.async channels.")
+
+(defprotocol PStore
+  (put-doc! [store doc]
+    "Write a document to the store. doc is a map:
+       {:id str, :value any, :version int, :updated int, :deleted bool, :synced bool}
+     Returns a channel that closes on success.")
+
+  (docs-by-prefix [store prefix]
+    "Get all documents whose id starts with prefix.
+     Returns a channel yielding a vector of doc maps.")
+
+  (get-meta [store key]
+    "Get a metadata value by key.
+     Returns a channel yielding the value, or nil if not found.")
+
+  (set-meta! [store key value]
+    "Set a metadata value. Returns a channel that closes on success.")
+
+  (close-store! [store]
+    "Close the store and release resources."))