ADR 0019: Define the PTO predicate-register contract

Historical context

This decision identified that every visible predicate needs explicit reset, preservation, producer, and consumer rules. The current PTO contract retains only P0 through P7: P0 is hardwired all ones, P1 through P7 are independently trap-preserved, and no accepted instruction consumes them. Machine-block encodings and their former execution-mask model are outside PTO.