Document kanta.ctx as the app-context channel for logemit

kanta.ctx already existed as a user-writable namespace for migrations;
since LogEvent carries the kanta instance, ev.kanta.ctx is also the way
for applications to pass per-connection metadata to their logemit
callbacks, including for creation/bootstrap events. No API change needed.

tests/test_logemit.py

@@ -344,3 +344,18 @@ def test_header_is_settable_and_used_by_default_emit(capsys):
     err = capsys.readouterr().err
     assert "CUSTOM update" in err
     assert "counter" in err  # default diff routing still applies
+
+
+@pytest.mark.asyncio
+async def test_ctx_reachable_from_event(tmp_path, format_config):
+    path = tmp_path / "test.db"
+    kanta = make_kanta(path, Data, format_config)
+    kanta.ctx.connection_id = 7
+    seen = []
+    kanta.logemit(lambda ev: seen.append(ev.kanta.ctx.connection_id) or True)
+    await kanta.open()
+    with kanta.transaction(action="inc") as data:
+        data.counter = 1
+    await kanta.close()
+
+    assert seen and all(connection_id == 7 for connection_id in seen)