Sequence verifiable units

Every step ends somewhere a check can confirm.

• Before the first change, run the work's checks on the starting base and record the results, so a later failure can be classed as pre-existing or as a regression.
• Split the work into small units that each end in a checked state, and start the next only when the current one passes. In a declared migration group the group is the unit (principle-migrate-without-shims).
• Where commits are authorized, keep each small and about one thing.
• Where commits are authorized, order them so the history proves the work: the failing test before the fix, removal before reshaping, baseline before treatment, scaffold before feature, each commit standing on its own.