Add DNS as a first-class multiplexed command, uniform with CommandConnections:
- command.go: CommandDNS constant (next in iota)
- command_client.go: case CommandDNS in dispatchCommands; DNSIncludeAnswers
option field (like StatusInterval); WriteDNSQuery in CommandClientHandler
All three upstream touch-points wrapped in // lx:begin dns / // lx:end dns
(CONSTITUTION 3.3). SPEC 018 v2.