feat(common): MNP 0.2 — liveness, and chat history read the way it is written

`get_messages` pages forward from the oldest message. That is the right shape
for "what happened since I last looked" and the wrong one for opening a
conversation, and the browser asked it for `since=0, limit=200` — so a group
with more than two hundred messages showed its first two hundred and the
exchange anyone came for was unreachable. Demonstrated on 300 messages: the
newest was simply absent from the answer.

`get_recent` and `get_before` page backwards, cursored on the row id rather than
the timestamp. Nothing makes a `time.time()` float unique, and a cursor on a
value two rows can share eventually skips a message or repeats it.

PING/PONG covers liveness on an already-open channel: a DataChannel whose peer
vanished without closing still reads as connected, and nothing noticed until a
real request hung. It is not a discovery mechanism — opening a connection to
ping costs a full ICE/DTLS handshake, measured at 0.6-7 s across two ISPs — so
presence in the group list comes from the hub's registry instead.

Both additions are backward compatible: an 0.1 peer sends no `before` and is
answered with the newest page, which is what it wanted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
index ed3925d..73e668c 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
@@ -203,6 +203,11 @@ class _MNPServerProtocol(QuicConnectionProtocol):
                 self._do_stream_segment_sync(stream_id, msg)
             elif mtype == MNP.CHAT_MESSAGE:
                 self._do_chat_message_sync(stream_id, msg)
+            elif mtype == MNP.PING:
+                # Liveness is transport-agnostic, and a native client over QUIC
+                # has the same half-open problem a DataChannel does.
+                self._send(stream_id, {"type": MNP.PONG, "v": MNP_VERSION,
+                                       "token": msg.get("token")})
             else:
                 log.warning("Unknown MNP message type: %s", mtype)
         except Exception as e: