### An optimisation stops and goes on to the same bytes

A long optimisation ran to its end in one process or not at all, and one killed partway lost every step.

- **wai-quantum:** `engine::variational::optimize_resumable` (and `call_optimize`, as `call` answers) goes on from a
  checkpoint, `wai.quantum.optimize-checkpoint/1`.
  - After every step, the caller gets the run as it stands, and may stop it there.
  - The checkpoint holds the steps taken with their links and the optimiser's whole state, every number as the bits
    of its f64.
  - Gone on with from any step, a run ends with the same bytes as one that never stopped, for each optimiser.
  - A checkpoint of another request, or one whose steps were changed, is refused (`J052`).
- **wai-quantum-cli:** `optimize --checkpoint FILE` keeps the state in FILE as the run goes, at most once a second and
  whenever it stops, written whole or not at all. It goes on from FILE when FILE is there, and removes FILE when the
  run ends. A run killed and started again with the same arguments ends with the same bytes, receipt and all.
  `--slice N` takes at most N steps, then stops with the state kept (exit 2).

