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/store/mod.rs

@@ -96,6 +96,32 @@ impl Store {
         index::all_keys(db)
     }
 
+    /// Get neighbors of a node: (key, strength) pairs.
+    pub fn neighbors(&self, key: &str) -> Result<Vec<(String, f32)>> {
+        let db = self.db.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
+
+        let uuid = match index::get_uuid_for_key(db, key)? {
+            Some(u) => u,
+            None => return Ok(Vec::new()),
+        };
+
+        let edges = index::edges_for_node(db, &uuid)?;
+        let mut neighbors = Vec::new();
+
+        for (other_uuid, strength, _, _) in edges {
+            // Look up key for other_uuid
+            let offsets = index::get_offsets_for_uuid(db, &other_uuid)?;
+            if offsets.is_empty() { continue; }
+            match capnp::read_node_at_offset(offsets[0]) {
+                Ok(n) if !n.deleted => neighbors.push((n.key, strength)),
+                _ => continue,
+            }
+        }
+
+        Ok(neighbors)
+    }
+
     /// Remove a node from the index (used after appending a tombstone).
     pub fn remove_from_index(&self, key: &str, uuid: &[u8; 16]) -> Result<()> {
         if let Some(db) = self.db.as_ref() {