Index of the coarsest child that is ready AND fresh for version, or -1
when none is fresh yet (the ≤1-frame window right after a re-slice, before
even the coarse level recommits). Children are stored coarsest→finest, so the
first match is the coarsest. The caller falls back to the coarsest READY
level on -1 so the group shows stale-but-ready geometry rather than blank.
Index of the coarsest child that is ready AND fresh for
version, or-1when none is fresh yet (the ≤1-frame window right after a re-slice, before even the coarse level recommits). Children are stored coarsest→finest, so the first match is the coarsest. The caller falls back to the coarsest READY level on-1so the group shows stale-but-ready geometry rather than blank.