Relay: viewers keep the file size (headers within 4 s, connection: close), retries back off, 412 re-resolves

Two server behaviours left ffmpeg (8.1/9.0) without the file size, after
which every seek failed with "size -38" until a reload:
- dart:io holds response headers until body bytes exist: a stalled
  source meant no headers within mpv's 5 s timeout. Relay byte responses
  are now written on the raw connection, headers with the first bytes or
  after 4 s, and a swap ends waiting reads with a plain close.
- every response closed its socket without saying connection: close, so
  ffmpeg 8.x reused a dead connection for its next seek (pol's X clips).
  The server now says it whenever the request asks.
A failed pull waits 250 ms doubling to 8 s instead of re-pulling at once
(the ~130 ms storm that drew Bilibili's 412/403); 412 counts as an expired
URL and re-resolves the page, at most once per 60 s. The app reloads a
player once per media when ffmpeg reports the lost size anyway. P37: a
read that waits past ffmpeg's retry budget still ends the file early.

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

docs/DEVELOPMENT.md

@@ -227,11 +227,12 @@ Traps:
   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`, `relay_e2e`): a viewer's
-  read is still in flight at the `stop` in tearDown.
+  response (one cut short at `stop`: `/m`, `/j`; the relay's byte
+  answers are written on the raw connection and end on their own) 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`, `relay_e2e`): a viewer's read can still be
+  in flight at the `stop` in tearDown.
 - The Link page probes a filled-in link (network, yt-dlp), which never
   answers on the widget-test clock: pass `AppShell(linkProbe:)` /
   `LibraryTab(probe:)` a fake, or `pumpAndSettle` never returns.