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/PROTOCOL.md

@@ -1,4 +1,4 @@
-# Protocol (proto = 12)
+# Protocol (proto = 13)
 
 Wire format only. Behavior (gate, clock, timeouts) is in
 [ARCHITECTURE.md](ARCHITECTURE.md). Source of truth:
@@ -21,7 +21,8 @@ with different `proto` refuse each other at `hello` (`proto mismatch`).
 - `GET /health` returns JSON (`ok`, `name`, `proto`, `viewers`, `bytesServed`,
   `uptimeSecs`; unauthenticated). `GET /m/<sid>/<token>/<name>`
   serves the host's file (Range table in ARCHITECTURE.md).
-  `GET /j/<sid>/<token>/<itemId>` relays the current Jellyfin media (see
+  `GET /j/<sid>/<token>/<itemId>` relays the current Jellyfin media and
+  Up next's first one (see
   "Jellyfin media" below). `/r/<sid>/<token>/...` is the relay of a
   member's media (see "Relay" below). `GET /a/<sid>/<token>/<hash>` serves
   a member's picture (see "Avatars" below), `GET /i/<sid>/<token>/<hash>`
@@ -30,7 +31,7 @@ with different `proto` refuse each other at `hello` (`proto mismatch`).
 ## Client to server
 
 ```
-{"t":"hello","proto":12,"secret":"","user":{"name":"","avatar":""},
+{"t":"hello","proto":13,"secret":"","user":{"name":"","avatar":""},
  "media":{"name":"","size":0,"duration":0,"url":null},"hostKey":"...",
  "resume":"<token>"}
 {"t":"ready","ready":true,"source":"manual"}      source: manual | auto
@@ -133,7 +134,8 @@ Field notes:
  "media":{name,size,duration,url?,jellyfin?,relay?,swapBy?,art?}|absent,
  "members":[{"id","name","avatarHash"?,"ready","inSet","buffered","stale","jf"?,
    "host"?,"out"?,"error"?,"rate"?}],
- "seekBy":"m1","seekSeq":3,"relay":true,"queue":[<item>]?,"queuePlay":"q3"?}
+ "seekBy":"m1","seekSeq":3,"relay":true,"relayNext":true,"queue":[<item>]?,
+ "queuePlay":"q3"?}
 {"t":"pong","c":1767225600000,"s":1767225600012}
 {"t":"feed","actor":"","action":"","target":"","reason":"","at":"<ISO-8601>"}
 {"t":"chat","text":"...","from":"<name>"}
@@ -159,7 +161,9 @@ Field notes:
   member left, and for the media the server started with). `media.art`:
   see "Media art". `seekBy` (member id) and `seekSeq` are present only while the current
   `eventId` came from a seek. `relay` (omitted when false): the server can
-  relay the current Jellyfin media. `members[].jf` is the member's last
+  relay the current Jellyfin media. `relayNext` (omitted when false, proto
+  13): it can relay Up next's first item too (a Jellyfin ref on the same
+  login), so viewers without access preload it from `/j`. `members[].jf` is the member's last
   `report.jf`, omitted when it never sent one. `members[].avatarHash`
   (omitted when the member has no picture) names the picture's bytes (see
   "Avatars"). `members[].host` (omitted when false): the member sent the
@@ -243,12 +247,13 @@ state   media.art = {"poster":"<hash>","banner":"<hash>"}
   without one plays the relay URL when `state.relay` is true: `welcome.relayUrl`,
   or after a swap built like the `/m` one from the host it dialled:
   `http://<host>/j/default/<token>/<itemId>`.
-- `GET /j/<sid>/<token>/<itemId>`: the server fetches
+- `GET /j/<sid>/<token>/<itemId>`: for the current item or, when
+  `state.relayNext`, Up next's first one, the server fetches
   `/Videos/<itemId>/stream?static=true&mediaSourceId=<id>` (the original
   file) with its own credential and streams it through. `Range` is forwarded
   as sent; status, `Content-Range`, `Content-Length`, `Content-Type` and
   `Accept-Ranges` come back as Jellyfin sent them. Errors: 401 bad token,
-  403 not the current item, 404 no relay for the current media,
+  403 neither of those items, 404 no relay for either,
   410 unknown session, 502 Jellyfin unreachable or rejecting the server's
   credential. Bytes count toward `bytesServed`.
 - Pasted Jellyfin links sent as plain `url` stay plain links.