M3 namespace migration: complete Phase B, partial Phase D, docs and tests

- Native runtime under bearhub/ (api, commons, gems, view/util, view/core, view/qt)
- bauh/ compatibility re-export shims for legacy imports
- Phase D: canonical __version__/__app_name__ in bearhub/__init__.py; pyproject.toml
  and setup.py point at bearhub package
- Native gem loader at bearhub/view/core/gems.py
- Namespace compatibility tests (159 tests passing)
- Documentation: CHANGELOG [Unreleased], NAMESPACE_MIGRATION, ROADMAP, README,
  CONTRIBUTING, docs/qt6-migration.md, packaging/aur/README.md

CONTRIBUTING.md

@@ -23,8 +23,10 @@ The way to go here is to ask yourself if the improvement would be useful for mor
 
 # Adding a new translation
 * To add a new translation, you will have to create a file in each directory listed below named as the first two letters of the language in the ISO format (e.g: for 'english' would be 'en'):
-- `bearhub/view/resources/locale` (or `bauh/view/resources/locale` during namespace migration)
-- `bauh/gems/appimage/resources/locale`
-- `bauh/gems/arch/resources/locale`
-- `bauh/gems/flatpak/resources/locale`
-- `bauh/gems/web/resources/locale`
+- `bearhub/view/resources/locale`
+- `bearhub/gems/appimage/resources/locale`
+- `bearhub/gems/arch/resources/locale`
+- `bearhub/gems/flatpak/resources/locale`
+- `bearhub/gems/web/resources/locale`
+
+New code should use `bearhub.*` imports. Legacy `bauh.*` imports still work via compatibility shims (see `NAMESPACE_MIGRATION.md`).