The TTML/DFXP reader ignored begin/end/dur on <body>, so a body begin
offset was lost and captions landed early. Body and div are now treated
as par time containers: their begins offset descendants, their ends
clip them, paragraphs without an end last until the container ends,
and end + dur take the earlier. seq containers warn instead of being
silently mistimed.
Fixes#895
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
