Pin chapter timescale when remuxing long movies

With FFmpeg 9.0.1, an automatic MP4 movie timescale of 6,000,000 made a long chapter exceed packet-duration limits; the resulting QuickTime chapter track had wrong timing despite a successful remux. Explicitly setting -movie_timescale 1000 fixed the chapter track without re-encoding video or audio. When the first source chapter starts late, a leading chapter at zero preserves the existing marker at its original time. Validate chapter start/end times and copied media hashes, not just chapter counts or the process exit code.