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_paste_shortcut.py

@@ -139,7 +139,7 @@ class TestYdotoolTerminalPasteCommand(unittest.TestCase):
     def test_flatpak_uses_shift_keycodes(self, mock_which, _mock_map):
         mock_which.return_value = "/app/bin/ydotool"
         injector = _make_injector()
-        with patch.dict("os.environ", {"FLATPAK_ID": "com.vocalinux.Vocalinux"}):
+        with patch.dict("os.environ", {"FLATPAK_ID": "com.voculinux.Voculinux"}):
             self.assertEqual(
                 injector._ydotool_ctrl_v_command(terminal=True),
                 ["ydotool", "key", "29:1", "42:1", "47:1", "47:0", "42:0", "29:0"],