Reserve a guest's share of a table bill and open its checkout

Reserves the requested share (full, equal, items or custom) against the table's live bill and opens a MyFatoorah checkout for it. The first share held for the current party, paid or still being paid, fixes the mode for the rest of the party (refusal splitModeMismatch otherwise), and for equal also the way-count: any other ways value is refused as splitWaysMismatch, even after earlier shares were paid, so a paid split's remainder is never re-split. A slot already held is refused as equalSlotClaimed. While the table's open party is one order, the till's own split parts on it count as held shares too: a Split equally part holds its slot and a By items part holds its lines (refusal itemAlreadyClaimed for a line it took), and either fixes the mode as a guest's share does. With several open orders at the table the till's parts are its order's own and are not counted. An optional tip (a venue QR preset by its key, or a custom amount in cents) is priced on the reserved share and charged with it, never inside it; it is refused as tipsOff where the venue's QR tips are off and tipChanged for a preset the venue no longer offers, and a changed tip opens a new session.