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/ops.rs

@@ -28,13 +28,12 @@ impl Store {
         Ok(())
     }
 
-    /// Add a relation (appends to log + updates cache + indexes)
+    /// Add a relation (appends to log + indexes)
     pub fn add_relation(&mut self, rel: Relation) -> Result<()> {
         self.append_relations(std::slice::from_ref(&rel))?;
         if let Some(db) = &self.db {
             index::index_relation(db, &rel.source, &rel.target, rel.strength, rel.rel_type as u8)?;
         }
-        self.relations.push(rel);
         Ok(())
     }