boot is the generation when this instance started. Nothing before it is known, so a holder
of an older generation resets. A change recorded earlier at this generation, by an instance
that restarted, is not known either; it falls inside the window between a save's first write
and its flush, when the page cache itself may still hold the old page.
The cursor at generation: the latest change inside it, or its floor when none was recorded.
Never below the vector's own floor, or a holder told to reset would be told again at its next
sync.
every tag changed after from, as of the object's current generation
records a change to tags inside generation; past the cap it degrades to a wholesale change
Everything changed. With advance the change takes a cursor of its own, for a purge that moved
no generation; without it the floor is the generation's own, for a bump that already did.
Tag to the cursor of its last change, with a floor under which nothing is known.
The floor is where this instance started, or where a wholesale change last landed. A holder whose cursor is below it is told to reset, because the tags that changed in between were either never recorded here or all changed at once. That is the conservative answer: a reset costs a device its copies and never shows it a stale page.