_poll_once() resets the parity timestamp when it runs with no subscribers so
the next subscription gets a fresh projection, but since the idle-park change
(#7694) the poll loop parks before calling _poll_once() once the last
subscriber leaves. A tab that reconnects inside the parity interval then
resumes with the stale cheap fingerprint and skips the projection.
Reset _last_cheap_fp and _last_full_projection_at in unsubscribe() when the
last subscriber is removed (not for an unknown queue, not while others
remain).