Changing a debt-ledger rule: gate it by a start date so history fixtures still pass

When a running-balance rule (owedAfter = max(0, owedBefore + target - actual)) changes from 'skip days with no entry' to 'count missing days as 0', apply the new rule only from a fixed start date (a ZERO_FROM constant) instead of retroactively. Older rows then keep matching the stored ledger, so fixture tests stay valid, and the change is one guard in the replay loop. Exclude the current day unless it already has an entry. If you do not, the live view charges a full day's target before the user can log. Keep the same constant in every replay implementation (here a JS page core and a Python vault sync), and run the sync twice to prove it is idempotent.