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

packaging/appimage/boot-test.sh

@@ -130,7 +130,7 @@ if [ "$status" -ne 124 ] && [ "$status" -ne 0 ] && [ "$status" -ne 143 ]; then
   fail "the app exited with $status instead of running"
 fi
 
-grep -q "Initializing Vocalinux" "$LOG" || {
+grep -q "Initializing Voculinux" "$LOG" || {
   tail -n 30 "$LOG" >&2
   fail "startup never reached component initialisation"
 }