Restructure step 5: one media pipeline on a player host

- PlayerHost port (MediaKitHost): players by role, pins, hwdec policy,
  render probe, log routing, disposal order; the only media_kit user.
- MediaPipeline: plan (one pure switch over state, origin, caps,
  presence) -> resolve -> open (prepared player promoted, else fresh)
  -> ready; commands queued during a switch apply once (P15); viewers
  play the buffer and wait for a dropped source without seek loops (D6).
- Every web page resolved by our yt-dlp into an EDL with headers and
  cookies (danmaku excluded); mpv's ytdl_hook no longer used (ytdl=no).
- D11: sign-in errors say which cookies were used and whether they hold
  any for the site.
- Relay: 256 KiB slices read as they fill, superseded pulls released
  (seek-to-play ~4.2 s -> ~2.8 s, P29); own Jellyfin relay channel only
  when someone needs it (P28).
- STARSYNC_SMOKE run and a Windows CI smoke step after the build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -533,7 +533,11 @@ export (picked or dropped; every site in it is kept). They stay on this
 computer and are used by its yt-dlp only (your own playback, "through
 me", and a server you host). Cookies log yt-dlp in as you on those
 sites: keep the file private, and for YouTube consider a throwaway
-account (Google may flag accounts used with yt-dlp).
+account (Google may flag accounts used with yt-dlp). When a page shows
+no video, the message says which cookies were used: none, your file
+(and whether it has any for that site), or your browser's (and how many
+it has for that site), so you know whether to sign in there or add
+cookies.
 YouTube works in all three too, and so do pages and links that only
 offer streaming playlists (HLS `.m3u8`, DASH `.mpd`, the HLS of X videos).
 Through me and Server fetches it never show the link to anyone; they can't pass on live