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_system_default_capture.py

@@ -31,7 +31,7 @@ def audio(monkeypatch: pytest.MonkeyPatch) -> Mock:
     audio.open.return_value.get_read_available.return_value = 1024
     audio.open.return_value.read.return_value = b"\x00\x01" * 1024
     monkeypatch.setitem(sys.modules, "pyaudio", SimpleNamespace(PyAudio=lambda: audio, paInt16=8))
-    monkeypatch.setitem(sys.modules, "numpy", sys._vocalinux_real_numpy)
+    monkeypatch.setitem(sys.modules, "numpy", sys._voculinux_real_numpy)
     return audio