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>

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