Create a Formation from an accepted hypothesis

Creates a Formation from a hypothesis (the subgroup proposal returned by POST /audiences/{audienceId}/formations/preview, optionally edited) and starts sorting the Audience's members into its subgroups. Returns immediately with status: building; poll GET /audiences/{audienceId}/formations/{formationId} until ready or failed. When the build runs inline the returned status may already be terminal. Any Audience viewer may create a Formation: collaborators create shared Formations, other viewers create ones private to themselves. The Audience must have at least one member and at most 1000. JSON bodies are limited to 64 KiB. Hypothesis text fields are trimmed and must be non-empty; subgroup IDs and labels must be unique. Reserved auxiliary IDs (unanswered, __unanswered, __other) cannot be used. User-created Formations require at least two populated primary buckets to become ready. All responses are private and non-cacheable.