Protocol 14: a warm relay preloads relayed Up next items for everyone

The server keeps one small relay (32 MiB ahead, 8 behind) for Up next's
first relayed item, keyed by its queue id at a URL that stays the same
after the swap: fetched by the server for server-fetch links, fed by
the adder's app for through-me links and files, by a member with an
account for member-passed Jellyfin items. Every app preloads from it;
the swap promotes it with its buffer. Dropped when the item stops
being first or its adder leaves. Measured switch: ~1.6 s cold, 20-40 ms
preloaded. ROADMAP gains the research item on server-side fetching of
member-resolved URLs.

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

README.md

@@ -167,9 +167,12 @@ or an episode on the Schedule for **Add to queue** / **Play next**; the
 Jellyfin details panel has a queue button (right-click it to play next),
 and the card for a dropped or pasted video or link has both. A link is
 checked first, like Put it up. Every app keeps the next item ready,
-its first seconds already loaded, so skipping to it starts at once
-(not yet for a link passed on, someone else's file or one the server
-fetches: those load when they go up). Everyone shares one Up next and anyone can change it; what
+its first seconds already loaded, so skipping to it starts at once.
+For a link passed on, a member's file or a link the server fetches,
+the server starts getting the next item while the current one plays
+(from the app of whoever added it, or by fetching the link itself), so
+everyone has its start before it goes up; whoever added it keeps the
+app open for that. Everyone shares one Up next and anyone can change it; what
 you added stays when you leave.
 
 In the player, Up next sits in the top bar next to the chat button: the