add ACTION message type and capability signaling

tests/test_codec.py

@@ -1,5 +1,5 @@
 from rrcd.codec import decode, encode
-from rrcd.constants import T_MSG
+from rrcd.constants import T_ACTION, T_MSG
 from rrcd.envelope import make_envelope, validate_envelope
 
 
@@ -9,3 +9,11 @@ def test_codec_round_trip() -> None:
     decoded = decode(data)
     assert decoded == env
     validate_envelope(decoded)
+
+
+def test_codec_round_trip_action() -> None:
+    env = make_envelope(T_ACTION, src=b"peer", room="#general", body="waves")
+    data = encode(env)
+    decoded = decode(data)
+    assert decoded == env
+    validate_envelope(decoded)