Swift Concurrency Runtime

Make ownership, capacity, and shutdown explicit, then prove behavior under adversarial timing.

Runtime contract

For each unit of work, identify:

• owner, lifetime, cancellation source, and completion observer;
• mutable-state owner and invariants crossing await;
• numeric capacity and queue/coalesce/replace/drop/reject policy;
• success, failure, cancellation, timeout, supersession, and shutdown semantics.

Keep these invariants: