Runtime use of type_check_only classes should give an error

Feature
Currently Mypy doesn't seem to care whether a class is marked with type_check_only in a stub file. My proposal is that any runtime use of such a class should produce an error, including importing it outside of if TYPE_CHECKING and using it in annotations without quotes or from __future__ import annotations.

Pitch
To create stubs for libraries that define classes dynamically, it's pretty much required to create classes in stubs that don't exist at runtime. Marking these with type_check_only lets you show that these classes can't be used at runtime, but consumers of your stubs could easily still do so accidentally. It would be useful if Mypy could warn them in those cases.

Start by tracing how mypy represents type_check_only classes in stubs and how it analyzes imports and annotations at runtime. Confirm the intended behavior for imports outside TYPE_CHECKING and unquoted annotations, then add coverage showing both cases produce errors.