store: internal locking, remove Arc<Mutex<Store>> wrapper

Store now has internal Mutex for capnp appends and AtomicU64 for
size tracking. All methods take &self. The external Arc<Mutex<Store>>
is replaced with Arc<Store>.

- Store::append_lock protects file appends
- local.rs functions take &Store (not &mut Store)
- access_local() returns Arc<Store>
- All .lock().await calls removed from callers

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/learn.rs

@@ -330,7 +330,7 @@ where
     let store_arc = crate::hippocampus::access_local()?;
 
     {
-        let store = store_arc.lock().await;
+        let store = &*store_arc;
         for (i, node) in context.conversation().iter().enumerate() {
             if let Some(key) = memory_key(node) {
                 if !seen.insert(key.to_owned()) { continue; }