feat(bus): identity.* topic family (formed / observation.linked / merged)

Fourth of the five-step identity-resolution substrate. Constants and
builder ship now; no publishers exist yet — they land with the
clusterer worker. Subscribers (webhook worker, dashboard SSE relay)
can register against identity.> from day one.

* decnet/bus/topics.py — IDENTITY root + IDENTITY_FORMED /
  IDENTITY_OBSERVATION_LINKED / IDENTITY_MERGED leaves; identity()
  builder mirroring the attacker() / system() helpers. Module
  docstring topic-tree updated.
* tests/bus/test_topics.py — assert builder produces the expected
  three topic strings + rejects empty event_type.

Wiki Service-Bus.md and a new Identity-Resolution.md page land in the
companion wiki-checkout commit.

tests/bus/test_topics.py

@@ -72,3 +72,17 @@ def test_system_control_builder() -> None:
 def test_system_control_rejects_bad_segments(bad: str) -> None:
     with pytest.raises(ValueError):
         topics.system_control(bad)
+
+
+# ─── Identity resolution topics (commit 4 of IDENTITY_RESOLUTION.md) ─────────
+
+
+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"
+
+
+def test_identity_builder_rejects_empty() -> None:
+    with pytest.raises(ValueError):
+        topics.identity("")