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_installer_engine_config.py

@@ -1,7 +1,7 @@
 """Behavioral tests for the installer's speech-engine sanity check.
 
 config.json survives reinstalls, so it can name an engine the current venv
-cannot import — vosk is an optional extra since #705. Vocalinux then dies at
+cannot import — vosk is an optional extra since #705. Voculinux then dies at
 startup with "No module named 'vosk'" instead of the installer saying so.
 """