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>

docs/DEVELOPMENT.md

@@ -56,7 +56,8 @@ and `e2e_check` (simulated viewers against a live server).
   `seek_drag_e2e`, `end_of_file_e2e`, `readahead_e2e`, `jellyfin_e2e` (a
   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,
+  fake Jellyfin and link host: an episode, a direct link, a link passed
+  on and a server-fetched one; `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
@@ -77,6 +78,11 @@ Traps:
   the `flutter_tester` by PID). `AppShell(videoBuilder:)` takes a stand-in,
   so shell tests run the real player layout without it; otherwise test pure
   logic (`StateDiff`, coordinator without widgets).
+- A `SyncServer` started inside a test: relic reports an error writing a
+  response (a relay response cut short when its relay is dropped at a
+  swap or stop) to the zone the server started in, which fails the
+  test; outside tests it is only logged. Start it in `runZonedGuarded`
+  and fail on anything else (`queue_switch_e2e`).
 - Never depend on a server-side WebSocket `onDone`; assert presence with
   deadline polling.
 - `sub.asFuture()` can resolve before `onDone`; poll with a deadline.