Fix cancel knit, move first run from `OpKnit::knit()` to `OpKnit::begin()`

[t0mpr1c3]

Closes #156. Also https://github.com/AllYarnsAreBeautiful/ayab-firmware/issues/166#issuecomment-1747233301. Depends on #154 and following PRs, except #163.

100% test coverage.