AgateClient takes address= default; attach_local accepts
address/host/gateway_ip per call with join control. An explicit
address skips the SoftAP radio join by default (join=True forces
it). derive_hotspot_credentials and join_device_hotspot accept a
gateway_ip override. battery/power host defaults now use
DEVICE_GATEWAY_IP. Verified live at 192.168.1.142:9000.