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>

packages/server/lib/src/sync_server.dart

@@ -101,7 +101,7 @@ class SyncServer {
     );
     _startedAt = DateTime.now();
     _heartbeat = Timer.periodic(const Duration(seconds: 1), (_) {
-      hub.engine.sweepStale();
+      hub.engine.tick();
       hub.sweepClosedSockets();
       hub.dropSilentSockets();
       hub.broadcastState();