Correcting the cookies theory — ytdl-sub is just CLI yt-dlp (no browser). The real
differentiators are a CURRENT yt-dlp + not fighting YouTube's client identity. We
were pinning a static user_agent in the yt-dlp opts, which yt-dlp recommends against
(it trips YouTube's heuristics and truncates large-playlist pagination). Dropped it
so yt-dlp manages its own (current) client — the lever that lets a fresh ytdl-sub
page further than us. (_UA is still used for the InnerTube requests.) Softened the
partial-playlist note to just 'Showing N of TOTAL videos.'
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