store: convert more callers to use RELS index

Convert remaining Vec users to index-based access:
- memory.rs: MemoryNode::from_store uses Store::neighbors()
- graph.rs: orphan detection uses for_each_relation
- local.rs: normalize_strengths uses for_each_relation + set_link_strength

Add Store::neighbors() method and index::get_offsets_for_uuid().

Cleanup:
- for_each_relation: build both uuid↔key maps in one pass
- cap_degree: consolidate key/uuid/degree collection

Remaining Vec uses: admin.rs (fsck, dedup), capnp.rs (load path).

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/hippocampus/graph.rs

@@ -882,18 +882,18 @@ pub fn health_report(graph: &Graph, store: &Store) -> String {
         .count();
 
     // Orphan edges: relations referencing non-existent nodes
+    // With index-based lookup, we count edges where endpoints don't resolve
     let mut orphan_edges = 0usize;
     let mut missing_nodes: HashSet<String> = HashSet::new();
-    for rel in &store.relations {
-        if rel.deleted { continue; }
-        let s_missing = !store.contains_key(&rel.source_key).unwrap_or(false);
-        let t_missing = !store.contains_key(&rel.target_key).unwrap_or(false);
+    store.for_each_relation(|source, target, _, _| {
+        let s_missing = !store.contains_key(source).unwrap_or(false);
+        let t_missing = !store.contains_key(target).unwrap_or(false);
         if s_missing || t_missing {
             orphan_edges += 1;
-            if s_missing { missing_nodes.insert(rel.source_key.clone()); }
-            if t_missing { missing_nodes.insert(rel.target_key.clone()); }
+            if s_missing { missing_nodes.insert(source.to_string()); }
+            if t_missing { missing_nodes.insert(target.to_string()); }
         }
-    }
+    });
 
     // NodeType breakdown
     let mut type_counts: HashMap<&str, usize> = HashMap::new();