store: index ops take WriteTransaction, mutations batch properly

Index functions now take &WriteTransaction instead of &Database,
allowing callers to batch multiple index operations in a single
transaction. Store mutations (upsert, delete, rename, etc.) now
begin_write/commit their own transactions, ensuring atomicity.

- replay_relations uses single txn for all relation indexing
- Store::db() exposes Database for callers needing txn control
- Convenience wrappers open their own txn for simple cases

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

src/hippocampus/store/capnp.rs

@@ -354,12 +354,14 @@ impl Store {
             }
         }
 
-        // Index relations directly (no Vec intermediate)
+        // Index relations directly (single transaction)
         if let Some(db) = &self.db {
+            let txn = db.begin_write()?;
             for rel in by_uuid.into_values() {
                 if rel.deleted { continue; }
-                index::index_relation(db, &rel.source, &rel.target, rel.strength, rel.rel_type as u8)?;
+                index::index_relation(&txn, &rel.source, &rel.target, rel.strength, rel.rel_type as u8)?;
             }
+            txn.commit()?;
         }
         Ok(())
     }