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

tests/api/abstract/test_model.py

@@ -1,6 +1,6 @@
 from unittest import TestCase
 
-from bauh.api.abstract.model import PackageUpdate
+from bearhub.api.abstract.model import PackageUpdate
 
 
 class PackageUpdateTest(TestCase):

tests/api/test_namespace.py

@@ -0,0 +1,17 @@
+from unittest import TestCase
+
+from bearhub.api.paths import CONFIG_DIR, CACHE_DIR
+from bearhub.commons.util import size_to_byte
+
+
+class TestApiNamespace(TestCase):
+
+    def test_bauh_compat_api_import(self):
+        from bauh.api.paths import CONFIG_DIR as legacy_config
+
+        self.assertEqual(legacy_config, CONFIG_DIR)
+
+    def test_bauh_compat_commons_import(self):
+        from bauh.commons.util import size_to_byte as legacy_size
+
+        self.assertEqual(legacy_size(1, 'K'), size_to_byte(1, 'K'))