The Iron Culture channel lists recent episodes mid-list (Ep 392/393 at
~position 360 of 396), so the 'channels enumerate newest-first' boundary
assumption is wrong: with it, rssd would break at the first pre-start-date
video and miss recent episodes, while re-resolving a boundary video every
poll. Walk channels exactly like playlists: always record out-of-range
videos as skipped so discovery stays incremental.
Also ignore media files (manual yt-dlp downloads have landed in the
working tree before).
