Function select_enrollment_capacity 

pub const fn select_enrollment_capacity(
    request: &EnrollmentRequest,
    current: EnrollmentCapacityCounters,
) -> EnrollmentCapacityDecision

Applies the enrollment runtime-capacity order atomically.

The order is identity Server then identity Conversation — the complete refusable set. A refusal exposes only the first full scope; success carries the post-increment identity counters and the fresh participant’s two reserved windows.