The previous detector substring-matched "TFF"/"BFF" against idet's own
label text, so it returned true on every source, and `cmd.Start()` was
never paired with `Wait()`, leaving a zombie ffmpeg per file.
Run idet bounded with `-frames:v 400 -an -sn -f null -` so it completes,
use CombinedOutput so the child is reaped, and parse the "Multi frame
detection" summary line — interlaced only when TFF+BFF > Progressive.
Esa Kataja