Output file path
Chapter definitions with title and start time (in seconds).
start is canonical, but startSec is also accepted because that is the
name used by ChapterMeta and is what callers naturally reach for.
Passing neither produces a clear error instead of writing START=NaN.
Chapter title
Optionalstart?: numberStart time in seconds
OptionalstartSec?: numberStart time in seconds (alias for start)
Optionalbinaryffmpeg binary override
Input file path