Whether a value covers everything beneath its path.
This is the structural coverage axis, and only that. How far to trust what is
covered is Source, and the two are deliberately independent: a cached value
covers the whole subtree but may be out of date, while a half-built one covers less
than the subtree but every byte in it was just observed.
An enum rather than a boolean because coverage will gain more ways to be incomplete
— truncated by a cap, cancelled, failed — and ordered worst-last so roll-ups combine
by taking the maximum. Those variants are not here yet; see the progressive-results
plan for the lifecycle they belong to.
Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.
The value does not account for everything beneath this path.
Not a promise of monotonicity. A value being built by an additive walk only
grows, but one left incomplete by reconciliation errors can move either way once
the missing part is read. Monotonicity is a property of the producer that is
running, not of this status, and a consumer that needs it must know a walk is in
progress rather than infer it from here.