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_auto_pause_monitor.py

@@ -678,7 +678,7 @@ class TestAutoPauseConfigDefaults(unittest.TestCase):
         from vocalinux.ui.config_manager import ConfigManager
 
         with tempfile.TemporaryDirectory() as tmp:
-            config_dir = os.path.join(tmp, "vocalinux")
+            config_dir = os.path.join(tmp, "voculinux")
             config_file = os.path.join(config_dir, "config.json")
             os.makedirs(config_dir, exist_ok=True)