checkMetadataConsistency used to live as a single unit under the global catalog.

New checks — including authoritative shard catalog — have grown that logic well beyond the global catalog. It is hard to review, and it no longer matches module or ownership boundaries.

We need to split this work into layers. The commands and the dispatcher should live in one place. And then, each catalog (global catalog, shard catalog, and so on) exposes APIs to check its own consistency, and the dispatcher calls those APIs.