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_playback_ducker.py

@@ -36,7 +36,7 @@ def _numpy_is_real_while_comparing() -> Iterator[None]:
     ``isinstance(value, numpy.bool_)`` and raises TypeError.
     """
     current = sys.modules.get("numpy")
-    real = getattr(sys, "_vocalinux_real_numpy", None)
+    real = getattr(sys, "_voculinux_real_numpy", None)
     if isinstance(current, MagicMock) and real is not None:
         sys.modules["numpy"] = real
         try:
@@ -620,7 +620,7 @@ def test_pactl_restores_each_channel(tmp_path: Path) -> None:
 def test_playback_duck_percent_clamps_and_round_trips(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None:
-    config_dir = tmp_path / "vocalinux"
+    config_dir = tmp_path / "voculinux"
     config_dir.mkdir()
     config_file = config_dir / "config.json"
     monkeypatch.setattr("vocalinux.ui.config_manager.CONFIG_DIR", str(config_dir))