Sync fixes: clean seeks, clock offset, end of file, 2.5 min read-ahead (proto 6)

- User seeks are intercepted at the player (SyncPlayer), previewed
  locally, sent once 300 ms after the drag ends; own echoes are matched
  by seekSeq and never re-applied; the landed check can't reapply a
  stale target. 1 s position poll removed.
- ping/pong clock offset (min-RTT ClockSync); positions use server time.
- Reports carry duration; buffer gate is min(4 s, remaining - 2 s);
  new finished transport instead of a HELD stall at the end.
- cache/readahead 150 s, 256 MiB demuxer cache, pins read back.
- Resumed reports put a stale ready member back in the set; hello.resume
  takes over the old member after an unclean drop (no ghost).
- Docs: roadmap rendering findings (HW path works; shaders verified).

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

apps/desktop/test/session_logic_test.dart

@@ -1,19 +1,6 @@
 import 'package:flutter_test/flutter_test.dart';
-import 'package:sync_relay_core/core.dart';
 import 'package:watch_together/src/session/idle.dart';
 import 'package:watch_together/src/session/session_coordinator.dart';
-import 'package:watch_together/src/sync/state_diff.dart';
-
-MemberView _self({bool ready = true, bool inSet = true, bool stale = false}) =>
-    MemberView(
-      id: 'm1',
-      name: 'me',
-      avatar: '',
-      ready: ready,
-      inSet: inSet,
-      buffered: 5,
-      stale: stale,
-    );
 
 void main() {
   group('IdleTracker', () {
@@ -104,17 +91,4 @@ void main() {
       expect(n.contains('SECRET'), isFalse);
     });
   });
-
-  group('shouldReassertReady', () {
-    test('ready, fresh, out of set (after stale sweep) -> reassert', () {
-      expect(shouldReassertReady(_self(inSet: false)), isTrue);
-    });
-    test('normal states -> no', () {
-      expect(shouldReassertReady(null), isFalse);
-      expect(shouldReassertReady(_self()), isFalse);
-      expect(shouldReassertReady(_self(ready: false, inSet: false)), isFalse);
-      expect(shouldReassertReady(_self(ready: false)), isFalse);
-      expect(shouldReassertReady(_self(inSet: false, stale: true)), isFalse);
-    });
-  });
 }