connect/disconnect/status, list_commands/command_info, the generic
send_command, get_state/watch_state/preflight_check, camera snapshot and
record, FTP listing and fetch, the read-only shell, and arm/disarm.
The safety gate is tested exhaustively rather than by sample: every motion
and envelope command is asserted to refuse while unarmed, and Landing and
Emergency are asserted to work without arming, because refusing to bring
down an airborne aircraft is the more dangerous answer.
preflight_check reads the per-element sensor keys, so a single failing
self-test is visible instead of being hidden by whichever arrived last.
