Add kanta instance to LogEvent and make header settable

Every emitted event now carries the originating Kanta instance so logemit
callbacks can reach application state attached to it. The header property
gains a setter, formalizing restyle-then-delegate: assign ev.header and
return truthy to keep the default diff routing with a custom header.

tests/test_logemit.py

@@ -315,3 +315,32 @@ def test_event_header_covers_all_kinds():
     )
     assert "transaction aborted: boom" in aborted.header
     assert "alice" in aborted.header
+
+
+@pytest.mark.asyncio
+async def test_event_carries_kanta_instance(tmp_path, format_config):
+    path = tmp_path / "test.db"
+    kanta = make_kanta(path, Data, format_config)
+    events = []
+    kanta.logemit(lambda ev: events.append(ev) or True)
+    await kanta.open()
+    with kanta.transaction(action="inc") as data:
+        data.counter = 1
+    await kanta.close()
+
+    assert events
+    assert all(ev.kanta is kanta for ev in events)
+
+
+def test_header_is_settable_and_used_by_default_emit(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+
+    def restyle(ev):
+        ev.header = f"CUSTOM {ev.action}"
+        return True
+
+    emit_event(_change_event(diff={"counter": 1}, previous={}), [restyle])
+    err = capsys.readouterr().err
+    assert "CUSTOM update" in err
+    assert "counter" in err  # default diff routing still applies