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/view/core/test_namespace.py

@@ -0,0 +1,23 @@
+from unittest import TestCase
+
+from bearhub.view.core.config import CoreConfigManager
+from bearhub.view.core.controller import GenericSoftwareManager
+from bearhub.view.core.gems import load_managers
+from bearhub.view.core.update import check_for_update
+
+
+class TestViewCoreNamespace(TestCase):
+
+    def test_bauh_compat_import(self):
+        from bauh.view.core.config import CoreConfigManager as legacy_config
+
+        self.assertIs(legacy_config, CoreConfigManager)
+
+    def test_controller_importable_from_bearhub(self):
+        self.assertTrue(callable(GenericSoftwareManager))
+
+    def test_gems_loader_native(self):
+        self.assertTrue(callable(load_managers))
+
+    def test_update_check_callable(self):
+        self.assertTrue(callable(check_for_update))