BT-UP-06: close() previously aborted the lifetime and closed the checkpoint
store immediately, so a caller could not wait for an active operation's terminal
settlement and a late provider result could still race the store.
The runtime now tracks every admitted operation until it settles. close() stays
the compatibility facade that closes admission and starts the drain, while
dispose() returns that same single-flight promise: it aborts the operation
registry, awaits actual settlement, and only then closes the checkpoint store
and cancellation channel. lifecycle() exposes OPEN, CLOSING and CLOSED, and a
draining runtime refuses new admission.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
