store: wire up RELS index for relations

Complete redb schema with bidirectional relation indexing:
- RELS multimap: uuid → packed(other_uuid, strength, rel_type, is_outgoing)
- Each edge stored twice (once per endpoint) with direction bit
- pack_rel/unpack_rel for 22-byte packed format

Wired up:
- replay_relations indexes all relations on load
- add_relation indexes new relations
- for_each_relation reads from index (graph building)
- add_link uses index for existence check
- set_link_strength finds/updates edges via index
- cap_degree uses index for degree counting and pruning
- rename_node finds edges by uuid

Vec<Relation> still maintained for remaining uses (normalize_strengths,
graph_health diagnostics). To be removed in follow-up.

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

src/hippocampus/store/capnp.rs

@@ -362,6 +362,13 @@ impl Store {
         self.relations = by_uuid.into_values()
             .filter(|r| !r.deleted)
             .collect();
+
+        // Index relations in redb
+        if let Some(db) = &self.db {
+            for rel in &self.relations {
+                index::index_relation(db, &rel.source, &rel.target, rel.strength, rel.rel_type as u8)?;
+            }
+        }
         Ok(())
     }
 
@@ -629,13 +636,16 @@ fn rebuild_index(db_path: &Path, capnp_path: &Path) -> Result<redb::Database> {
         let txn = database.begin_write()?;
         {
             let mut nodes_table = txn.open_table(index::NODES)?;
-            let mut uuid_table = txn.open_table(index::UUID_TO_KEY)?;
+            let mut key_uuid_table = txn.open_table(index::KEY_TO_UUID)?;
+            let mut uuid_offsets = txn.open_multimap_table(index::UUID_OFFSETS)?;
 
             for (key, (offset, uuid, _, deleted)) in latest {
                 if !deleted {
                     nodes_table.insert(key.as_str(), offset)?;
-                    uuid_table.insert(uuid.as_slice(), key.as_str())?;
+                    key_uuid_table.insert(key.as_str(), uuid.as_slice())?;
                 }
+                // Always record offset in UUID history (even for deleted)
+                uuid_offsets.insert(uuid.as_slice(), offset)?;
             }
         }
         txn.commit()?;