you stated that the spacecraft and the astronaut and the flashlight
were traveling at c
so there must be a point somewhere that the speed measurement is
being taken from as the spacecraft moves away from it at c.
and were only talking about the light from the flashlight I suppose.
so an observer traveling outside the spacecraft at c would see the same
thing that the astronaut inside the spacecraft sees if there is a portal
that he can look through , they both would see the light shining onto the
front of the craft from the back of the craft.
and if light detection devices are placed along the path of the craft then
the light detectors should detect the same thing as the astronaunt and the
outside observer are able to see given that the detectors are properly designed
and calibrated to detect higher and lower frequency light because the craft
will be moving towards and away from the detectors at c as the craft passes
by them and the light from the flashlight will be traveling 1 x faster
than c as the craft is approaching the detectors and 1 x faster than c as
the craft is moving away from the detectors.
so the detectors would need to adjust for the changes in light frequency
that enters into the detection portions of the detectors.
like the red shift blue shift.
if we focus on what our eyes can see then we end up with false
math and a magic show to boot to sell books.https://www.youtube.com/watch?v=KJN5w9bMleM