implement-phase

Implement exactly one phase of a signed-off spec on its own branch (or as one commit series on a long-lived branch, in rebase style) — Phase 0 first, which changes no behaviour — keeping the diff inside the phase, flagging anything the spec didn't cover instead of silently deciding it, and recording deviations as an "As built" list. Use when a spec has been signed off and it's time to write code, or when resuming a phase.