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/gems/appimage/test_namespace.py

@@ -0,0 +1,22 @@
+from unittest import TestCase
+
+from bearhub.gems.appimage.controller import AppImageManager
+from bearhub.view.core.gems import _discover_gem_dirs, _import_controller_module
+
+
+class TestAppImageNamespace(TestCase):
+
+    def test_bauh_compat_import(self):
+        from bauh.gems.appimage.util import replace_desktop_entry_exec_command
+
+        self.assertTrue(callable(replace_desktop_entry_exec_command))
+
+    def test_appimage_discovered_under_bearhub(self):
+        gem_dirs = dict(_discover_gem_dirs())
+        self.assertIn('appimage', gem_dirs)
+        self.assertIn('bearhub/gems/appimage', gem_dirs['appimage'].replace('\\', '/'))
+
+    def test_controller_import_prefers_bearhub(self):
+        module = _import_controller_module('appimage')
+        self.assertEqual(module.__name__, 'bearhub.gems.appimage.controller')
+        self.assertIs(module.AppImageManager, AppImageManager)

tests/gems/appimage/test_util.py

@@ -1,6 +1,6 @@
 from unittest import TestCase
 
-from bauh.gems.appimage.util import replace_desktop_entry_exec_command
+from bearhub.gems.appimage.util import replace_desktop_entry_exec_command
 
 
 class TestUtil(TestCase):