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/single_instance.py

@@ -1,5 +1,5 @@
 """
-Single instance enforcement for Vocalinux.
+Single instance enforcement for Voculinux.
 
 Uses file locking to ensure only one instance runs at a time.
 Works across sessions and handles stale locks from crashed processes.
@@ -16,7 +16,7 @@ from .utils.paths import data_dir
 
 logger = logging.getLogger(__name__)
 
-# Lock file location: $XDG_DATA_HOME/vocalinux/ (defaults to ~/.local/share/vocalinux/)
+# Lock file location: $XDG_DATA_HOME/voculinux/ (defaults to ~/.local/share/voculinux/)
 LOCK_FILE_DIR = Path(data_dir())
 LOCK_FILE_PATH = LOCK_FILE_DIR / "instance.lock"
 
@@ -61,8 +61,8 @@ def acquire_lock() -> bool:
             fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
         except (OSError, IOError):
             # Lock is held by another process
-            logger.warning("Another instance of Vocalinux is already running. Exiting.")
-            print("Another instance of Vocalinux is already running.", file=sys.stderr)
+            logger.warning("Another instance of Voculinux is already running. Exiting.")
+            print("Another instance of Voculinux is already running.", file=sys.stderr)
             print(
                 f"If you're sure no other instance is running, delete: {LOCK_FILE_PATH}",
                 file=sys.stderr,