Chen's switching principle

Compatibility facade for the content-based modules in MathlibNt/SieveTheory/Switching/. All existing declarations remain in the MathlibNt.SieveTheory.SwitchingPrinciple namespace.

The sequence develops weights and representation counts, the corrected sieve and Pan counting bridge, Mertens main terms, weighted penalties, source sieves, dimension-one prime sums, logarithmic meshes, Rosser boundary comparisons, and endpoint assembly. Import this facade to retain the complete historical API, including its explicit analytic hypotheses and documented limitations.