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>

docs/ROADMAP.md

@@ -56,6 +56,25 @@ off by deleting it in the same commit as the code. Wire changes also edit
      over instead of the buffer running dry.
    - "Bob can't open this link — relay it?": offer the relay to whoever put
      a direct link up when a viewer's player fails to open it.
+   - Preload relayed Up next items (see ARCHITECTURE "Getting ready"):
+     today a link passed on "through me", another member's file, a
+     server-fetched link and the host's own file are opened cold by
+     everyone but their source when they go up (their relay or `/m`
+     starts with the swap), so the gate waits for the slowest open. Plan
+     (wire change): the server holds a second, small relay for Up next's
+     first item, keyed by its queue id. Its adder's app dials a second
+     source for it as soon as it is first (`/r/.../source` naming the
+     queue id); a server-fetched item starts its fetch then;
+     `state.queue[0].relay` (`live`, `tracks`) tells viewers when and
+     how to open `/r/<sid>/<token>/q/<queueId>/<name>` in their preload
+     player. The buffer keeps only the head and tail pins plus about
+     32 MiB ahead of the viewers until the swap, pulls nothing while
+     nobody preloads, and is dropped when the item stops being first. A
+     swap with `queueId` turns it into the media's relay (same buffer;
+     the `q/` URL keeps working for that media, so the preloads match
+     and are promoted). The host's file: the hosting app hands the
+     server the first item's path early and `/m` serves it under a
+     `next/` name until the swap.
 
 8. **Jellyfin** (epic, below).