[caglarpir]

Problem

Interval sampling stamps each frame with the video's start time plus its time in the video. Each second of a timelapse spans several seconds of capture, so its frames are stamped that many times too close together:

• With --geotag_source gpx, the frames all land near the start of the track. A 5 s, 30x GoPro MAX timelapse, geotagged from a GPX of its own GPS track, comes out with a median error of 670 m, its frames spread over 21 m of a 1.4 km route.
• With --geotag_source gopro, the frames are placed right but stamped 0.5 s apart for 15 s of travel, so the speed check rejects them (1,134 km/h).

Changes

• Interval sampling reads the rate from the video's GPS clock with _read_video_clock(), as GPX sync does since Sync a GPX to the GPS clock of a timelapse, even one too noisy to publish #849, and stamps the frames at that rate. The start time is mapped back from the first GPS timestamps at the same rate.
• --video_duration_ratio used to apply to every video. It now paces only the videos without a GPS clock, so a ratio given for those doesn't affect the other videos in a batch. A video whose clock overrides a given ratio logs a warning naming the clock's rate.
• GeotagImagesFromVideo (the gopro, camm and blackvue image sources) times the video's track in GPS seconds at the clock's rate, to match the stamps. This also fixes distance samples of a timelapse, which were already stamped with their capture times.
• The telemetry is read once per video. If it fails to parse, the warning is now "Unable to read the telemetry of ...".

Verification

Two GoPro MAX timelapses, remuxed to a single video stream (see the end), each geotagged from a GPX of its own GPS track, interval 0.5 s. Median error against the video's own track:

	main	this PR
30x, gpx	670 m	0.5 m
15x, gpx	85 m	0.1 m
30x, gopro	rejected (speed)	0 m
15x, gopro	0 m	0 m

• Distance samples with gopro: 148 of 150 and 81 of 82 frames geotagged, where main geotags 0 and 7.
• Normal-speed videos (GoPro HERO7 and HERO8, BlackVue, Insta360 Pro), interval sampling with gpx and with their own source: output identical to main.
• Unit: 919 passed, 18 skipped, 1 xfailed. Integration: 69 passed. Doctests, mypy, ruff check, ruff format --check and usort diff are clean.

GoPro MAX .360

Interval sampling interleaves the two video streams of a .360 file, so its frames are stamped as if twice as late (#854 fixes that). Until then, on a .360 timelapse the second half of the frames is stamped past the end of the track and fails with MapillaryOutsideGPXTrackError, where main placed every frame 127 m to 1.4 km off.

With #854: one hunk in _sample_single_video_by_interval: probe once, then this PR's telemetry and rate. With #851: the imports of sample_video.py.