Compatibility import for the generic pair builder

The implementation now belongs to the machine-model subroutine layer. Import Complexitylib.Models.TuringMachine.Subroutines.PairBuild in new code; all Complexity.TM names, exact machines, and bounds are unchanged.

This legacy path remains for downstream imports and the old SAT-specific GuessVerify route. Remove it only in an announced breaking release after those clients have migrated; it must never own a second implementation.