Make ctraits.c free-threading friendly

`ctraits.c` does not currently support free-threaded builds of Python, so an import of `traits` on a free-threaded build of Python 3.14 will re-enable the GIL. We should update `ctraits.c` to support free-threaded builds.

This entails reworking the module initialization and reworking several pieces of currently global module state.

This is a fairly major task, and I don't anticipate anyone Enthought-side having the resources to carry it out in the near future. Help would be welcome.

Key reference: https://github.com/python/cpython/blob/main/Doc/howto/free-threading-extensions.rst