Re-runs and corrections

A processed month is never edited; it is re-run. The new run supersedes the old, history keeps both, and the audit log shows the correction.

• Free pool — each month includes a number of re-runs; beyond it, re-runs bill at half rate and are funded by carried credits first, then the month's unused credit, then balance.
• Merges — processing a second group into an open month is a merge, not a re-run.
• Rectification approvals trigger re-runs when the approver chooses so.
• Arrears instalments, open earnings, reimbursements and OT all survive a re-run.