build_runtimes() logs-and-skips a camera it cannot open, so a wrong
/dev/video index or an unseated ribbon produced a green systemd unit
that silently monitored one camera less.
- edge/probe.py: opens each camera in CAMERAS, retries while the sensor
settles, reports frame geometry, optionally saves preview PNGs, and
exits non-zero with a backend-specific hint (v4l2-ctl / rpicam-hello)
- setup_online.sh: new step [8/9] runs it against the installed package
and the CAMERAS line actually written to edge.env, and refuses to
enable the unit on failure. SKIP_CAMERA_CHECK=1 opts out.
Tests: 174 pass, 2 skipped (7 new).
