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

src/vocalinux/utils/snap.py

@@ -17,6 +17,6 @@ SNAP_GPU_NOTE = (
 
 
 def running_in_snap(environ: Optional[Mapping[str, str]] = None) -> bool:
-    """Whether this process runs inside the Vocalinux snap."""
+    """Whether this process runs inside the Voculinux snap."""
     env = os.environ if environ is None else environ
     return bool(env.get("SNAP_NAME") or env.get("SNAP"))