Keep pythonx a namespace package: rule and publish check #106

The maintainer's instruction (2026-10-04, relayed by the ecosystem lead): every pythonx library installs under pythonx.*, so they must install side by side. pythonx is a namespace package, and no wheel may ship pythonx/__init__.py. pythonx-concurrent, pythonx-graphics and pythonx-platform already have the rule and the check.

Add three things here:

• the rule, in AGENTS §12;
• a test in tests/test_wheel.py;
• a publish-pypi.yml step that fails if the wheel contains pythonx/__init__.py.

Done when the test passes and the workflow carries the step.