Fix degrade extra-outputs key in fan-out example

The fan-out-with-retry degrade mode supplied its extra_outputs value
under the parent field name (`topics`) in degraded_update, but
proposal 0066 reads degraded_update in the subgraph's field-name space.
The subgraph field is `topic` (extra_outputs={topics: topic}), so the
parent-keyed entry was ignored, leaving the slot null (0069) and
tripping BatchState.topics: list[str] validation in degrade mode.

Key the degraded value by the subgraph field `topic` (the doc
walkthrough already documents `topic: other`). Caught running
MODE=degrade against the v0.14.0rc1 artifact; the example smoke test
only exercises fail_fast, so it slipped through #161.

Author: chris-colinsky

examples/fan-out-with-retry/main.py

@@ -336,11 +336,14 @@ def build_graph(mode: str = "fail_fast") -> CompiledGraph[BatchState]:
         # partial in place of the instance result, so the batch finishes
         # instead of aborting (fail_fast) or dropping the instance
         # (collect). Retry stays inner so it still sees raw transients
-        # first. The degraded mapping is keyed the way the fan-out
-        # projects an instance: the collect_field (``summary``) plus
-        # each parent extra_outputs key (``topics``).
+        # first. The degraded mapping is keyed in the subgraph's
+        # field-name space (proposal 0066): the collect_field (``summary``)
+        # plus each extra_outputs subgraph field (``topic``, which the
+        # fan-out projects to the parent ``topics`` list). Supplying
+        # ``topic`` keeps the slot non-null so the ``list[str]`` parent
+        # field validates (an omitted source would be a null slot, §9.3).
         degrade = FailureIsolationMiddleware(
-            degraded_update={"summary": "(unavailable)", "topics": "other"},
+            degraded_update={"summary": "(unavailable)", "topic": "other"},
             event_name="headline_degraded",
         )
         instance_middleware = (degrade, retry, timing)