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>

docs/HOW-IT-WORKS.md

@@ -47,7 +47,9 @@ Friends join either way with the Join card on Home.
    says we are, and buffers. A new arrival is not ready yet and never holds
    anyone back.
 3. **Ready gate.** Press Space to say you are ready. The server starts
-   playback only when everyone who is ready has at least 4 seconds buffered.
+   playback only when everyone who is ready has at least 4 seconds buffered
+   (or what is left, near the end). At the end everyone stops and the server
+   stays finished until someone seeks back or changes the video.
    If two or more people are ready, one person alone cannot start it for the
    others. A lone viewer can play alone.
 4. **Play.** The server owns the clock. Each app compares its mpv position
@@ -61,7 +63,7 @@ Friends join either way with the Join card on Home.
 7. **Leave.** Closing or leaving drops you from the server; the others keep
    going. If you vanish without saying goodbye, you are marked stale after 15 s
    (stops blocking) and removed after 30 s. The app reconnects by itself and
-   restores your ready flag.
+   takes over your old place (same member, ready flag and video).
 
 There are no roles: anyone can seek, swap, force-ready others and kick
 anyone except the host.