Get a Formation with its subgroups and members

Returns one Formation: build status, live build progress, the stored hypothesis, and the real subgroups with their member assignments and confidences. Poll this after POST /formations, POST /formations/preview acceptance, or recompute until status is ready or failed; while building, subgroups may be empty and hypothesis.subgroups describes the proposed buckets. Visible to any Audience viewer for shared Formations and to the creator for private ones. Responses are private and non-cacheable. Stored diagnostics are omitted; failed builds expose only safe failure reasons. Invalid legacy proposals become an empty object, malformed building progress becomes null, and ready builds always return null progress.