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/bus/topics.py

@@ -17,6 +17,7 @@ Token structure (NATS-style, dot-separated):
     identity.formed
     identity.observation.linked
     identity.merged
+    identity.unmerged
     credential.captured
     credential.reuse.detected
     system.log
@@ -101,12 +102,21 @@ ATTACKER_INTEL_ENRICHED = "intel.enriched"
 #   identity.merged              — two identities collapsed; loser gets
 #                                  ``merged_into_uuid`` set, subscribers
 #                                  re-key cached references to the winner
+#   identity.unmerged            — revocable-merge undo: contradicting
+#                                  evidence cleared ``merged_into_uuid``
+#                                  and re-split observations.  The
+#                                  resurrected side's UUID is the same
+#                                  as the prior loser, so subscribers
+#                                  that cached references to the loser
+#                                  during the merged interval can
+#                                  re-attach without a new lookup.
 #
 # ``identity.campaign.assigned`` is deferred; it ships when the campaign
 # clusterer ships.  YAGNI before then.
 IDENTITY_FORMED = "formed"
 IDENTITY_OBSERVATION_LINKED = "observation.linked"
 IDENTITY_MERGED = "merged"
+IDENTITY_UNMERGED = "unmerged"
 
 # Credential event types (second/third tokens under ``credential``).
 # ``credential.captured`` fires once per upserted Credential row — the
@@ -215,9 +225,9 @@ def identity(event_type: str) -> str:
     """Build ``identity.<event_type>``.
 
     *event_type* is typically one of :data:`IDENTITY_FORMED`,
-    :data:`IDENTITY_OBSERVATION_LINKED`, :data:`IDENTITY_MERGED`. Dotted
-    leaves (``observation.linked``) are permitted — same rationale as
-    :func:`system`.
+    :data:`IDENTITY_OBSERVATION_LINKED`, :data:`IDENTITY_MERGED`, or
+    :data:`IDENTITY_UNMERGED`. Dotted leaves (``observation.linked``)
+    are permitted — same rationale as :func:`system`.
     """
     if not event_type:
         raise ValueError("identity topic requires a non-empty event_type")

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:

tests/bus/test_topics.py

@@ -81,6 +81,7 @@ def test_identity_builder() -> None:
     assert topics.identity(topics.IDENTITY_FORMED) == "identity.formed"
     assert topics.identity(topics.IDENTITY_OBSERVATION_LINKED) == "identity.observation.linked"
     assert topics.identity(topics.IDENTITY_MERGED) == "identity.merged"
+    assert topics.identity(topics.IDENTITY_UNMERGED) == "identity.unmerged"
 
 
 def test_identity_builder_rejects_empty() -> None:

tests/clustering/test_clusterer_worker.py

@@ -149,6 +149,9 @@ async def test_publishes_cluster_result_on_bus(monkeypatch, repo):
         identities_merged=[
             {"winner_uuid": "id-1", "loser_uuid": "id-2"},
         ],
+        identities_unmerged=[
+            {"resurrected_uuid": "id-2", "former_winner_uuid": "id-1"},
+        ],
     )
     clusterer = _FakeClusterer(results=[result])
 
@@ -169,6 +172,7 @@ async def test_publishes_cluster_result_on_bus(monkeypatch, repo):
     assert _topics.identity(_topics.IDENTITY_FORMED) in topics_seen
     assert _topics.identity(_topics.IDENTITY_OBSERVATION_LINKED) in topics_seen
     assert _topics.identity(_topics.IDENTITY_MERGED) in topics_seen
+    assert _topics.identity(_topics.IDENTITY_UNMERGED) in topics_seen
 
 
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