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_single_instance_ext.py

@@ -183,7 +183,7 @@ class TestLockFileConstants(unittest.TestCase):
     """Tests for lock file constants."""
 
     def test_lock_file_path_is_in_data_home(self):
-        """Test that lock file lives under $XDG_DATA_HOME/vocalinux/."""
+        """Test that lock file lives under $XDG_DATA_HOME/voculinux/."""
         expected_dir = Path(data_dir())
         self.assertEqual(single_instance_module.LOCK_FILE_DIR, expected_dir)
         self.assertEqual(single_instance_module.LOCK_FILE_PATH, expected_dir / "instance.lock")