Remove pkg_resources version lookup

[Mirochill]

Summary

• Remove the import-time pkg_resources lookup from breadability.__version__.
• Keep the package version available without adding a new runtime dependency, preserving the existing old-Python compatibility surface.
• Add a small regression test for the exposed package version.

Fixes #39

Validation

• git diff --check HEAD~1..HEAD
• rg -n pkg_resources|get_distribution|DistributionNotFound|resource_filename|load_entry_point breadability tests setup.py requirements.txt (no matches)
• Not run locally: test suite