Whether this index placed any commit at all — false for the empty fallback.
True when oid is strictly later on the branch than than, false when it
is the same commit or earlier, and null when either commit is
unplaceable — the caller's cue to fall back to timestamps rather than to
treat "unknown" as "no".
Position of oid (0 at the branch tip), or null when the index never saw it.
Positions of a run's commits on its branch, newest first. Built once per observation; every comparison after that is a map lookup.