Arms with the live session's id; a pagehide before this is a no-op.
Disarms after the session ended some other way — the close is idempotent, but the pagehide stays quiet.
The IN-PAGE teardown signal: the element was removed from the page (an SPA navigation) with a call still up. The same decision a terminal pagehide makes. Returns whether a close was sent.
The pagehide decision. persisted is the event's bfcache flag — a persisted page may come back, so it
does NOT close. Returns whether a close was sent.
Whether a live session is armed.