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_settings_test_dictation.py

@@ -215,7 +215,7 @@ def test_test_dictation_cancel_runs_off_thread_and_restores_callbacks() -> None:
     dialog.test_button.set_label.assert_called_with("Cancelling…")
     thread_cls.assert_called_once()
     assert thread_cls.call_args.kwargs["daemon"] is True
-    assert thread_cls.call_args.kwargs["name"] == "vocalinux-test-dictation-cancel"
+    assert thread_cls.call_args.kwargs["name"] == "voculinux-test-dictation-cancel"
 
     with patch.object(settings_dialog, "GLib") as glib:
         thread_cls.call_args.kwargs["target"]()
@@ -443,7 +443,7 @@ def test_finalize_test_buffered_reload_waits_for_idle_before_restore() -> None:
     dialog.speech_engine.stop_recognition.assert_not_called()
     thread_cls.assert_called_once()
     assert thread_cls.call_args.kwargs["daemon"] is True
-    assert thread_cls.call_args.kwargs["name"] == "vocalinux-test-dictation-stop"
+    assert thread_cls.call_args.kwargs["name"] == "voculinux-test-dictation-stop"
     thread_cls.call_args.kwargs["target"]()
     dialog.speech_engine.stop_recognition.assert_called_once()
     # Must not restore live injector yet — worker still holds the utterance.
@@ -518,7 +518,7 @@ def test_cancel_during_processing_restores_once_without_result_check() -> None:
     ):
         SettingsDialog._finalize_test(dialog)
 
-    assert thread_cls.call_args.kwargs["name"] == "vocalinux-test-dictation-stop"
+    assert thread_cls.call_args.kwargs["name"] == "voculinux-test-dictation-stop"
     dialog._test_cancelled = True
     assert SettingsDialog._finish_test_after_user_cancel(dialog) is False
     assert dialog.test_buffer._text == "(Test cancelled)"