close
Promote edge to Lifecycle.CLOSING — no new delegation is admitted, but spend and release still drain it (design §5.3). null if edge is unknown/divergent, or already Lifecycle.RETIRED. Idempotent from closing.
Promote edge to Lifecycle.CLOSING — no new delegation is admitted, but spend and release still drain it (design §5.3). null if edge is unknown/divergent, or already Lifecycle.RETIRED. Idempotent from closing.