Protocol 11: split-track relay (YouTube video+audio), chunked upstream reads

Relayed media can carry 2-4 byte tracks (video, audio): yt-dlp picks a
split pair when no single-file format exists (h264 first), the source
channel serves each track, the server keeps a buffer per track (budget
split by size, 8 MiB floor) and viewers open an mpv edl:// of the track
URLs. Expired URLs re-resolve once for all tracks. Upstream GETs are cut
into <= http_chunk_size (10 MiB) ranges: YouTube throttles larger ones.
Live streams and DRM are refused; HLS/DASH pieces are a follow-up.

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

README.md

@@ -304,9 +304,9 @@ Web pages work in all three: the app and the dedicated server each
 download [yt-dlp](https://github.com/yt-dlp/yt-dlp) by themselves (checked
 against the release's checksums, updated once a day; Settings → Player
 shows its version and has "Update now"), so there is nothing to install.
-The last two need a page with a single-file video (X has one; YouTube only
-has separate video and audio streams, so use "Everyone fetches it") and
-never show the link to anyone. Your upload has to beat the video's
+YouTube works in all three too. The last two never show the link to
+anyone; they can't pass on live streams or pages that only offer streaming
+playlists (use "Everyone fetches it" for those). Your upload has to beat the video's
 bitrate: a 25-minute 2 GB episode is about 11 Mbit/s.
 
 ## Video quality and shaders