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

docs/ARCHITECTURE.md

@@ -1,6 +1,6 @@
 # Architecture
 
-Vocalinux is a GTK 3 desktop application. `main.py` wires together the
+Voculinux is a GTK 3 desktop application. `main.py` wires together the
 configuration manager, recognition manager, tray UI, D-Bus service, and text
 injector. Audio capture and recognition run outside the GTK event loop.
 
@@ -104,7 +104,7 @@ as `tray_indicator.py`, `dictation_overlay.py`, `transcription_history.py`,
 
 ## Services and optional integrations
 
-`dbus_service.py` owns the `com.vocalinux.Vocalinux` session-bus name and
+`dbus_service.py` owns the `com.voculinux.Voculinux` session-bus name and
 provides Toggle, Start, and Stop methods for external controls.
 `gateway_embed/` manages the optional local VocaGateway container lifecycle,
 status, pairing, and configuration. It is separate from the default