Protocol 13: Up next preloads, so switching to it is instant

Every app gets the first Up next item ready itself (own Jellyfin plan,
direct links, guests via /j when state.relayNext says the server's
login can pass it on) and opens it in a hidden, muted, paused second
player; the swap promotes that player instead of opening cold, so its
buffer counts at once. Reports go out on open and when the buffer
reaches the gate's need. SwitchTrace times each phase. Measured in the
e2e harness: 1.5-3 s before, 35-103 ms with a preload. Relayed items
still open cold for everyone but their source (ROADMAP).

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

README.md

@@ -166,8 +166,10 @@ right-click a Jellyfin card or episode, a video in Files, a recent link
 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. The next item is kept ready on the app
-that will put it up, so skipping to it starts at once. Everyone shares one Up next and anyone can change it; what
+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
 you added stays when you leave.
 
 In the player, Up next sits in the top bar next to the chat button: the