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>

apps/desktop/test/stream_lost_test.dart

@@ -0,0 +1,20 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:starsync/src/platform/ports/player_host.dart';
+
+// ffmpeg's lost stream (its HTTP context lost the file's size, every seek
+// fails until a fresh open): the lines the coordinator reloads the video
+// on, once per media.
+void main() {
+  test('a seek that failed for want of the size is a lost stream', () {
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'error', 'Seek failed (to 3417735, size -38)')), isTrue);
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'warn', 'Seek failed (to 11997, size -38)\n')), isTrue);
+  });
+
+  test('anything else is not', () {
+    // A seek that failed with the size known, a reconnect, another prefix.
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'error', 'Seek failed (to 20851, size 1023081)')), isFalse);
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'warn', 'http: Will reconnect at 3407872 in 0 second(s)')),
+        isFalse);
+    expect(mpvStreamLost(const PlayerLogLine('lavf', 'warn', 'Seek failed (to 1, size -38)')), isFalse);
+  });
+}