Pacing: the crew's concurrency and dispatch rate follow each subscription's remaining window, and a paused successor says the real reason #373

Part of #341, after subscription standing.

• Concurrency (crew/runner.ts, crew/autonomy.ts slots) rises toward the remaining window capacity and falls as a wall approaches, never past the operator's headroom.
• A run paused at a wall resumes at the reset automatically (it already re-queues); its successor's label names the real reason (which account, which window, when it resets) instead of a generic pause.
• Stickiness: a run never changes subscription mid-flight — rotation happens only between runs (today's takes over path already makes a new run; keep it so, with a test).
• The policy is a pure function (standing in, slots and next dispatch time out), tested with hand-worked cases.