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/DEVELOPMENT.md

@@ -39,7 +39,9 @@ the dedicated binary. Prints `READY port=... name=...` and stops on SIGINT
 or SIGTERM. Then Join from one or more apps at `127.0.0.1:8080`. Or just
 Host from the app. mpv env override:
 `SYNC_RELAY_HWDEC` (wins over the Settings choice). `SYNC_RELAY_VO` has no
-effect with embedded video: media_kit forces `vo=libmpv`.
+effect with embedded video: media_kit forces `vo=libmpv`. `SYNC_RELAY_PRELOAD=0`
+turns Up next's preload player off; `SYNC_RELAY_TRACE_SWITCH=1` prints
+the timings of each media change (`SwitchTrace`).
 
 Harnesses in `tool/` (own `pubspec.yaml`, `dart run tool/<name>.dart`):
 `range_logger` (serve a file and log every Range request), `shaper`
@@ -52,7 +54,10 @@ and `e2e_check` (simulated viewers against a live server).
 - `cd apps/desktop && flutter test test/<file>`.
 - Real-mpv tests: `sync_playback_e2e`, `coord_presence`, `video_reload_e2e`,
   `seek_drag_e2e`, `end_of_file_e2e`, `readahead_e2e`, `jellyfin_e2e` (a
-  local fake Jellyfin over HTTP; tests never talk to a real Jellyfin). The sync and
+  local fake Jellyfin over HTTP; tests never talk to a real Jellyfin),
+  `queue_switch_e2e` (times Up next switches against a delayed, rate-capped
+  fake Jellyfin and link host; `WT_SWITCH_PRELOAD=0` for the cold path,
+  `WT_SWITCH_DELAY_MS` / `WT_SWITCH_RATE_KBS` for the link). The sync and
   end-of-file ones expect `~/Videos/chiyo.mp4`; others generate media with
   ffmpeg into `WT_TEST_TMP`. Headless mpv (no video output attached) won't
   open video-only files, so test media needs an audio track. Tests always log