ReadonlybackHow many samples into its own past the reference is read — 0 is the same instant.
ReadonlyrefinedA trusted reading has already moved this aim onto its own arrival.
Carried because an aim must be refined ONCE and then left alone. Re-deciding it every run
would re-decide it on a number that jitters by a sample, and every move drops the frame run
— so a policy that keeps refining never lets a run finish, and the row reports
too-few-frames for ever while looking like it is working hard.
Where the reference window is being read, and whether that aim has been refined yet.