Add plain SciPy wrapper factory

[jonathanberthias]

The explicit-state path began with a handwritten Normal example. Repeating its constructor, parameter resolution, and evaluation code for each plain SciPy law would make the catalog hard to maintain. Keeping the SciPy implementation in distributions/core.py would also make every future backend depend on SciPy.

This PR adds a factory for opt-in continuous wrappers that expose SciPy's native shape names. It moves the concrete SciPy implementation and its parameter defaults to distributions/scipy_adapter.py, leaving backend-independent contracts in distributions/core.py. Normal uses the factory, while Gamma and Genextreme demonstrate required shape parameters.

Generated constructors have useful runtime signatures but looser static typing. The concrete adapter remains available as a precisely typed construction path until the public API cutover.