feat(batch-diff): side-by-side claim diff between two batches

backend/src/cyclone/store.py

@@ -1144,6 +1144,31 @@ class CycloneStore:
             rows = s.query(Batch).order_by(Batch.parsed_at.asc()).all()
             return [self._row_to_record(r) for r in rows]
 
+    def load_two_for_diff(
+        self,
+        a_id: str,
+        b_id: str,
+    ) -> tuple[BatchRecord, BatchRecord]:
+        """Load two batches by id for the side-by-side diff view.
+
+        Returns ``(a, b)`` as ``BatchRecord`` objects. Raises
+        :class:`LookupError` when either id is missing — the API layer
+        catches it and maps it to ``404 Not Found`` (matching the
+        ``GET /api/batches/{id}`` contract). The two loads happen in
+        independent sessions so a transient failure on one side can't
+        poison the other.
+
+        Used exclusively by :mod:`cyclone.batch_diff` via the
+        ``/api/batch-diff`` endpoint.
+        """
+        a = self.get(a_id)
+        if a is None:
+            raise LookupError(f"batch {a_id} not found")
+        b = self.get(b_id)
+        if b is None:
+            raise LookupError(f"batch {b_id} not found")
+        return a, b
+
     def iter_claims(
         self,
         *,