Send raw payloads over UDP and emit them as udp events

Review feedback from Cooldude2606: UDP is most useful for arbitrary
blobs, so send_udp now takes any string and the packet is just
"channel?payload".  Plugins get it as a udp-<channel> event with the
payload as a Buffer, or through handleUdp which logs handler errors
like handle does for ipc events.

The host socket is now opened when the RCON client is started and
closed where the RCON client is ended, matching how the RCON client is
handled, instead of being bound in start() and closed by _resetState.

The integration round trip moved to the FactorioServer tests, where it
sends a packet each way through a real server without the module.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

packages/host/lua/clusterio_lib/api.lua

@@ -73,9 +73,7 @@ function api.send_udp(channel, data)
 		return -- Lua UDP is not enabled for this instance
 	end
 
-	channel = escape_channel(channel)
-	data = compat.table_to_json(data)
-	helpers.send_udp(port, "\f$ipc:" .. channel .. "?j" .. data, 0)
+	helpers.send_udp(port, escape_channel(channel) .. "?" .. data, 0)
 end