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, 9 insertions, 1 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/protocol.py b/packages/meshbay-common/src/meshbay_common/protocol.py
index b5a54ff..1aadafe 100644
--- a/packages/meshbay-common/src/meshbay_common/protocol.py
+++ b/packages/meshbay-common/src/meshbay_common/protocol.py
@@ -27,8 +27,16 @@ class MNP:
     STREAM_SEGMENT  = "stream_seg"       # HLS/DASH segment
     CHAT_MESSAGE    = "chat_msg"         # Double Ratchet message
     CHAT_ATTACHMENT = "chat_attach"      # attachment metadata
-    CHAT_HISTORY    = "chat_hist"       # request message history
+    CHAT_HISTORY    = "chat_hist"       # request message history (newest, or before a cursor)
     CHAT_HISTORY_RESPONSE = "chat_hist_resp"  # history response with messages
+    # Liveness on an *already open* channel. A peer that goes away without
+    # closing leaves a DataChannel that still reads as connected until the next
+    # real request hangs, and there was no way to ask. This is not a discovery
+    # mechanism: opening a connection in order 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 socket registry instead.
+    PING            = "ping"
+    PONG            = "pong"
     # GEK_REQUEST / GEK_RESPONSE were removed (NS3, and finding L1): the node must
     # never serve the GEK in plaintext. Members obtain it by unwrapping their own
     # ECIES bundle. The constants lingered after the handlers were deleted, leaving