Split a customer transaction (create or replace its legs)

#876. Replaces every split leg and void-and-replaces the posted transaction journal entry with one category line per leg (the bank/card offset leg is preserved). Legs must sum exactly to the transaction amount; at least 2 legs. Legs may carry a classId. Acts as the accountant: no client action is queued for review. 422 when the transaction is reconciled, has an applied transfer/credit-card match, is matched to a bill/document, is on a personal bank account, or its entry is in a filing-locked period.