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_silero.py

@@ -22,7 +22,7 @@ from unittest.mock import MagicMock, patch  # noqa: E402
 # load and don't restore it. Reuse the real module cached by conftest instead
 # of unloading/re-importing NumPy's compiled extensions.
 if isinstance(sys.modules.get("numpy"), MagicMock):
-    _real_numpy = getattr(sys, "_vocalinux_real_numpy", None)
+    _real_numpy = getattr(sys, "_voculinux_real_numpy", None)
     if _real_numpy is not None:
         sys.modules["numpy"] = _real_numpy