Apologies for bothering a mailing list, but I have gone through a couple of
hundred Google URLs looking for an answer and done experiments/diagnosis for
a day. Usually, this process reveals the mis-understanding.
I have enabled udp6 transport in the configuration, and the resulting snmpd
indeed handles requests via the ipv6 address.
The socket/bind calls that handle udp6: apparently use in6addr_any, so I
would expect that snmpd would also handle an ipv4 request, but the debug
output says that those requests never reach snmp.