Restructure step 1: lasting identity, presence and protocol 16

Server and core: version ranges on both sides with a coded refusal; a
client id maps to the same member and token for the server's life; a
drop is `gone` for a 10-minute grace keeping ready, presence, queue
items, relay and avatar; `bye` leaves at once; ordered takeover (seq,
replaced/stale); presence joining/in/away/gone with one PresencePolicy;
joiners never pause playback and hold a held gate at most ~15 s;
state.hold; feed ids, seq and replay; the relay lives through its
source's grace and continues on re-dial; queue `by` lasting, auto
items and cancel; MediaRef carried; client ids never logged.

App: persisted client id and seq; Connection with unstable/lost/
replaced states, backoff with jitter, no lost/back pair per flap; a drop
keeps the session (state, player, media, relay source); presence in the
strip and members card; sit out = away intent; feed matched by id;
auto sit-out (idle when not playing, minimised for a while; never when
alone) replaces auto-unready; bye on close and Leave, not on update
restart; relay source re-dials through its grace.

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

docs/HOW-IT-WORKS.md

@@ -76,10 +76,12 @@ window; guests need no Jellyfin or account.
 6. **Swap.** Anyone can change the video by pasting another link. Everybody
    stops at 0 and the gate runs again for everyone there; ready flags stay
    as they were (a setting unreadies you instead).
-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
-   takes over your old place (same member, ready flag and video).
+7. **Leave.** Closing the app or Leave says goodbye and drops you from the
+   server; the others keep going. If you vanish without a goodbye, after 15 s
+   of silence you are "gone": you stop blocking but keep your place
+   (ready flag, sitting out, Up next items, the video you pass on) for
+   10 minutes. The app keeps playing and reconnects by itself, with an id
+   of its own, and takes that place back.
 
 There are no roles: anyone can seek, swap, force-ready others, nudge
 anyone, and sit anyone out (the host too). Someone sitting out stays