store: remove Vec<Relation>, dedup uses index iteration

The relations Vec is gone from Store. dedup now iterates via
edges_for_uuid() instead of mutating in-memory Vec — removes/re-adds
edges through the index directly.

Removed load_relations_vec() and clear_relations() — no longer needed.
Added helper methods: edges_for_uuid, index_relation, remove_relation_from_index.

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

src/hippocampus/store/index.rs

@@ -209,18 +209,6 @@ pub fn remove_relation(
     Ok(())
 }
 
-/// Clear all relations from the index.
-pub fn clear_relations(db: &Database) -> Result<()> {
-    let txn = db.begin_write()?;
-    {
-        // Drop and recreate the table
-        txn.delete_multimap_table(RELS)?;
-        let _ = txn.open_multimap_table(RELS)?;
-    }
-    txn.commit()?;
-    Ok(())
-}
-
 /// Get all edges for a node. Returns (other_uuid, strength, rel_type, is_outgoing).
 pub fn edges_for_node(db: &Database, node_uuid: &[u8; 16]) -> Result<Vec<([u8; 16], f32, u8, bool)>> {
     let txn = db.begin_read()?;