Progress with a denominator: the leaves of the plan tree.
Unlike Completion, progress carries serde: it is a reading of a
file that the interface renders, not a claim that work finished. What
must not be deserialisable is a Done — and that still is not.
Two figures, because either alone misleads. The share says how
much of the plan is behind you and moves when a branch is divided
generously; the leaf count says how many pieces the plan turned out
to have and does not. A reader seeing 83% / 4 leaves left can tell
the difference between work finished and work redistributed, and a
reader seeing one number cannot.