graph: use index for bulk reads, skip capnp deserialization

- Add all_keys() to StoreView, use in build_adjacency instead of
  for_each_node (which was ignoring content/weight anyway)
- Add all_key_uuid_pairs() for single-pass uuid mapping
- Extend KEY_TO_UUID to store [uuid:16][node_type:1][timestamp:8]
- for_each_node_meta now reads from index, no capnp needed
- Add NodeType::from_u8() for unpacking

Graph health: 7s → 2s (3.5x faster)

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

src/hippocampus/graph.rs

@@ -519,11 +519,9 @@ pub fn build_graph_fast(store: &impl StoreView) -> Graph {
 
 fn build_adjacency(store: &impl StoreView) -> (HashMap<String, Vec<Edge>>, HashSet<String>) {
     let mut adj: HashMap<String, Vec<Edge>> = HashMap::new();
-    let mut keys: HashSet<String> = HashSet::new();
 
-    store.for_each_node(|key, _, _| {
-        keys.insert(key.to_owned());
-    });
+    // Get keys directly from index — no need to deserialize node content
+    let keys: HashSet<String> = store.all_keys().into_iter().collect();
 
     store.for_each_relation(|source_key, target_key, strength, rel_type| {
         if !keys.contains(source_key) || !keys.contains(target_key) {