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_upgrade.py

@@ -359,7 +359,7 @@ run_interactive_install
 echo "U=$UPGRADE_MODE N=$NON_INTERACTIVE I=$INTERACTIVE_MODE"
 """)
 
-    assert "existing Vocalinux installation was detected" in result.stdout
+    assert "existing Voculinux installation was detected" in result.stdout
     assert "REACHED_GUIDED_SETUP" not in result.stdout
     assert "U=yes N=yes I=no" in result.stdout
 
@@ -407,7 +407,7 @@ def test_piped_installer_defaults_to_upgrade_on_existing_install(tmp_path: Path)
     venv = tmp_path / "venv"
     (venv / "bin").mkdir(parents=True)
     (venv / "bin" / "activate").touch()
-    config_dir = tmp_path / ".config" / "vocalinux"
+    config_dir = tmp_path / ".config" / "voculinux"
     config_dir.mkdir(parents=True)
     (config_dir / "config.json").write_text(json.dumps({"speech_recognition": {"engine": "vosk"}}))
 
@@ -432,7 +432,7 @@ def test_piped_installer_defaults_to_upgrade_on_existing_install(tmp_path: Path)
     env = os.environ.copy()
     env["HOME"] = str(tmp_path)
     # conftest points XDG_CONFIG_HOME at an isolation dir; the installer reads
-    # config.json from $XDG_CONFIG_HOME/vocalinux, not straight from $HOME.
+    # config.json from $XDG_CONFIG_HOME/voculinux, not straight from $HOME.
     env["XDG_CONFIG_HOME"] = str(tmp_path / ".config")
     env["XDG_STATE_HOME"] = str(tmp_path / ".local" / "state")
     env["TMPDIR"] = str(tmp_path)