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/mind/subconscious.rs

@@ -528,7 +528,7 @@ impl Subconscious {
 
             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,
             };
             for key in surface_str.lines().map(|l| l.trim()).filter(|l| !l.is_empty()) {
@@ -606,7 +606,7 @@ impl Subconscious {
         // Query each agent's recent writes so they know what they already touched
         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,
         };