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/qt/test_namespace.py

@@ -0,0 +1,19 @@
+from unittest import TestCase
+
+from bearhub.view.qt.prepare import PreparePanel
+from bearhub.view.qt.window import ManageWindow
+from bearhub.view.qt.components import QtComponentsManager
+
+
+class TestViewQtNamespace(TestCase):
+
+    def test_bauh_compat_import(self):
+        from bauh.view.qt.components import QtComponentsManager as legacy_manager
+
+        self.assertIs(legacy_manager, QtComponentsManager)
+
+    def test_prepare_panel_importable(self):
+        self.assertTrue(callable(PreparePanel))
+
+    def test_window_importable(self):
+        self.assertTrue(callable(ManageWindow))