Ship pythonx-map.toml inside the package and prove the wheel's contents #13

What

Move the manifest into the package directory (or fix package-data) and add a test that builds the wheel and lists it.

Why

SPEC S1.1: the manifest sits at the repository root, outside every package directory, so package-data cannot pick it up. AGENTS.md §15: a pattern is not proof.

Done when

A test builds the wheel and finds pythonx-map.toml, py.typed and the .pyi files inside it.