feat(identity): rename user-visible identity to voculinux with one-time legacy import

The fork takes its own name everywhere a user can see it: binaries
(voculinux, voculinux-gui), desktop entry, autostart, tray, D-Bus
namespace (com.voculinux.Voculinux, no alias for the old name),
config/data/cache dirs, single-instance lock, IBus engine identity,
tray icons, resource prefixes, VOCULINUX_* environment names, and the
update-checker user agent. The internal Python module stays vocalinux
and the distribution metadata keeps the upstream URLs as provenance.

On first run a previous Vocalinux install's config.json, custom
dictionary, and portal restore token are copied once into the new
locations (new config always wins, no merge, no write-back); models
re-resolve through the pinned-checksum download path. An offline
naming-sweep test proves no old-name strings remain on user-visible
surfaces outside the allowlist.

Refs #18

tests/test_recognition_manager_helpers.py

@@ -167,8 +167,8 @@ class TestGetSystemModelPaths(unittest.TestCase):
                 mock_open.return_value.read.return_value = mock_os_release
                 paths = _get_system_model_paths()
                 self.assertIsInstance(paths, list)
-                self.assertNotIn("/usr/local/share/vocalinux/models", paths)
-                self.assertIn("/usr/share/vocalinux/models", paths)
+                self.assertNotIn("/usr/local/share/voculinux/models", paths)
+                self.assertIn("/usr/share/voculinux/models", paths)
 
     def test_arch_paths_custom_xdg_dirs(self):
         # Regression test: custom XDG_DATA_DIRS + Arch os-release used to