Add cgo accessible logging shim for the custom ffmpeg log callback

[seanavery]

Description

We currently have coarse FFmpeg log handling: the cgo callback formats each line and dumps it to stderr/stdout via fprintf, and we rely on the viam-server parent process to interpret stderr as errors and stdout as info.

This PR changes the callback to hand the formatted line and level up to a Go shim, which routes it through the module's logging.Logger under an ffmpeg sublogger (libav levels map onto rdk loglevels).

Testing

• Unit tests verify mapping of FFmpeg loglevel to our rdk loglevels ✅
• Sanity check ffmpegLogger atomic.Pointer use by running tests with -race flag ✅
• test live on machine full stack with viamrtsp
  • confirm debug ✅
  • confirm info ✅
  • confirm error ✅

5/26/2026, 4:42:20 PM debug viamrtsp.ffmpeg     utils/utils_ffmpeg_log_cgo.go:94 [segment @ 0x7835c8000c00] -> pts:1445400 pts_time:16.06 dts:1445400 dts_time:16.06
5/26/2026, 4:42:20 PM debug viamrtsp.ffmpeg     utils/utils_ffmpeg_log_cgo.go:94 [segment @ 0x7835c8000c00] stream:0 start_pts_time:240.14 pts:23058000 pts_time:256.2 dts:23058000 dts_time:256.2
5/26/2026, 4:42:20 PM debug viamrtsp.ffmpeg     utils/utils_ffmpeg_log_cgo.go:94 [segment @ 0x7835c8000c00] -> pts:1449000 pts_time:16.1 dts:1449000 dts_time:16.1
5/26/2026, 4:42:20 PM debug viamrtsp.ffmpeg     utils/utils_ffmpeg_log_cgo.go:94 [segment @ 0x7835c8000c00] stream:0 start_pts_time:240.14 pts:23061600 pts_time:256.24 dts:23061600 dts_time:256.24
5/26/2026, 4:42:20 PM debug viamrtsp.ffmpeg     utils/utils_ffmpeg_log_cgo.go:94 [segment @ 0x7835c8000c00] -> pts:1454400 pts_time:16.16 dts:1454400 dts_time:16.16
``

[cheukt]

lgtm once tested on a real robot

[katiepeters]

Sweet! Thanks for doing this. Looks like it already has reviewer coverage, so I'm going to bow out

[seanavery]

@cheukt I had to move to NewModule to get the logger working. NewModuleFromArgs logger return seemed to be behaving differently, is that expected?

[cheukt]

ya NewModuleFromArgs creates a blank logger for you, so having to thread it through makes sense.