Function parseDuration

Parse a duration string to a number of seconds.

Accepts both forms ffmpeg emits:

• plain seconds, e.g. "120.042000" (ffprobe -print_format json)
• clock notation, e.g. "00:02:00.042" or "2:00" (ffprobe text output)

Returns null if the value is absent, "N/A", or unparseable.