CatalogCoverage

How much of the store the model actually accounts for.

Reported rather than assumed, because a model with a hole in it is worse than no model: a path that looks absent because its segment was never sketched is indistinguishable, to a caller, from a path that is genuinely absent. Two things can leave a hole — a segment whose sidecar has not been built yet, and documents that are still in the memtable — and each has its own number here.