Remove a Mind from an Audience

Removes one Mind from an Audience and from every Formation subgroup on that Audience. The Mind itself is not deleted. Owner only; collaborators and viewers receive 404. Ownership is rechecked under a transaction lock; membership and subgroup cleanup commit together. Responses are private and non-cacheable, with generic 500 errors for unexpected storage failures. Removing a Mind that is not a member is a no-op that still returns 204. In the background the Audience's allocation audit is measured again on the remaining members, and members whose training failed are retrained.