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/user/mod.rs

@@ -421,7 +421,7 @@ async fn run(
             }
             let store_arc = crate::hippocampus::access_local().ok();
             let store_guard = match &store_arc {
-                Some(s) => Some(s.lock().await),
+                Some(s) => Some(&**s),
                 None => None,
             };
             app.unconscious_state = unc.snapshots(store_guard.as_deref());