migrate local.rs and memory.rs to use index

- Add Store::all_keys() method for iteration
- Convert store.nodes.get() → store.get_node()
- Convert store.nodes.contains_key() → store.contains_key()
- Convert store.nodes.values() iteration → all_keys + get_node

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

src/hippocampus/store/mod.rs

@@ -94,6 +94,13 @@ impl Store {
         index::contains_key(db, key)
     }
 
+    /// Get all node keys.
+    pub fn all_keys(&self) -> Result<Vec<String>> {
+        let db = self.db.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
+        index::all_keys(db)
+    }
+
     pub fn resolve_key(&self, target: &str) -> Result<String> {
         // Strip .md suffix if present — keys no longer use it
         let bare = strip_md_suffix(target);