feat(bus): reserve identity.unmerged topic

Revocable merges (a contradiction-driven undo of identity.merged) ship
in the clusterer work; this reserves the topic up-front so identity.>
subscribers receive it day one without a re-subscribe.

The clusterer worker's ClusterResult fan-out now publishes on
identity.unmerged when populated. The skeleton clusterer never
populates it; the revocable-merge commit will.

Wiki update lives in wiki-checkout/Service-Bus.md (separate repo).

decnet/clustering/worker.py

@@ -148,9 +148,13 @@ async def _publish_result(bus: Optional[BaseBus], result: ClusterResult) -> None
             merged,
             event_type=_topics.IDENTITY_MERGED,
         )
-    # identities_unmerged ships once IDENTITY_UNMERGED is reserved
-    # (next commit). The field is already on ClusterResult so the
-    # revocable-merge work doesn't reshape the dataclass.
+    for unmerged in result.identities_unmerged:
+        await publish_safely(
+            bus,
+            _topics.identity(_topics.IDENTITY_UNMERGED),
+            unmerged,
+            event_type=_topics.IDENTITY_UNMERGED,
+        )
 
 
 async def _wake_on(bus: BaseBus, wake: asyncio.Event, pattern: str) -> None: