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_ibus_active_engine_guard.py

@@ -29,7 +29,7 @@ sys.modules["gi"] = _mock_gi
 sys.modules["gi.repository"] = _mock_gi_repo
 
 for _key in list(sys.modules.keys()):
-    if "vocalinux" in _key and "ibus_engine" in _key:
+    if "voculinux" in _key and "ibus_engine" in _key:
         del sys.modules[_key]
 
 import vocalinux.text_injection.ibus_engine as ibus_engine  # noqa: E402