Make the vision preflight detect a camera-less image

[RobVanProd]

The preflight could not fail

bridge/vision_service.py --preflight reported "ready": true unconditionally. It never contacted
the robot, so it could not distinguish a working setup from one that would never produce a single
detection — and tools/start_local_vision.ps1 gates on that result, so it would happily launch a
worker doomed to return nothing.

That is exactly F3's signature failure, and a large part of why it stayed undiagnosed. F3 reads
"the camera works and the robot is serving frames; the host has never returned a single detection."
A worker polling a camera-less image looks identical to a detector that simply sees nobody: zero
faces, no errors, forever.

The reference robot reports compiled_enable_camera: 0 and compiled_enable_camera_host_vision: 0.
The old preflight still called it ready.

Change

Preflight now reads the unauthenticated /debug surface and refuses when:

• either compiled camera flag is explicitly disabled → firmware-camera-disabled: <flags>
• the robot is unreachable → robot-unreachable: <error type>
• /debug is unparsable or not an object

It reports the distinguishing reason and exits 2, so the launcher stops rather than starting a
worker that cannot work.

An absent flag is treated as unknown rather than disabled. /debug truncates by omitting fields
(debug_response_truncated: true is observable on the live robot), so reading omission as "camera
off" would turn a large snapshot into a false negative.

Verified against the live robot

Before — same robot, camera compiled out:

{"schema":"stackchan.local-vision-start.v1","status":"preflight-ready", ...}   exit 0

After:

{"ready":false,"reason":"firmware-camera-disabled: compiled_enable_camera,compiled_enable_camera_host_vision", ...}
exit 2

and tools/start_local_vision.ps1 stops with Local vision preflight failed with exit 2.

Notes

• No new secret handling: /debug needs no pairing code, and the pairing code is still never
  fetched into the payload. The existing no-leak assertion is retained.
• bridge/test_vision_service.py: 11/11, covering ready, camera-disabled, unreachable, and the
  truncated-response case.
• The old test was named "…without fetching…"; preflight now deliberately does fetch, so that
  contract is replaced rather than worked around.

Pairs with #224, which supplies a camera profile that does not also switch motion on. Neither is
physical qualification of F3.