RunCoordinator stopping-owner handoff

Trap: awaiting a stopping owner with Effect.andThen looks like orderly serialization, but it propagates the interrupted owner’s failed Exit, so a forced run() registered during cleanup never starts its promised successor. Fix: mark the entry stopping before durable cancellation, preserve pendingWake, await completion as an Exit and discard the old result, then retry run(key); setup or cleanup failure must release waiters and permit exactly one successor.