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_language_shortcuts.py

@@ -106,7 +106,7 @@ def test_normalize_language_shortcuts_dedupes_by_shortcut() -> None:
 
 
 def test_language_shortcuts_roundtrip_through_config(tmp_path: Any) -> None:
-    config_dir = tmp_path / "vocalinux"
+    config_dir = tmp_path / "voculinux"
     config_dir.mkdir()
     config_file = config_dir / "config.json"
     with (
@@ -137,7 +137,7 @@ def test_language_shortcuts_roundtrip_through_config(tmp_path: Any) -> None:
 def test_get_language_shortcuts_survives_a_broken_shortcuts_section() -> None:
     """A hand-edited config replacing the section with a scalar reads as empty."""
     with tempfile.TemporaryDirectory() as temp_dir:
-        config_dir = os.path.join(temp_dir, "vocalinux")
+        config_dir = os.path.join(temp_dir, "voculinux")
         config_file = os.path.join(config_dir, "config.json")
         os.makedirs(config_dir, exist_ok=True)
         with open(config_file, "w") as handle:
@@ -1062,7 +1062,7 @@ def test_slovenian_shortcut_row_persists_through_config(
     dialog_class: type[Any], tmp_path: Any
 ) -> None:
     """A row picked as Slovenian is saved and read back, not dropped (#970)."""
-    config_dir = tmp_path / "vocalinux"
+    config_dir = tmp_path / "voculinux"
     config_dir.mkdir()
     with (
         patch("vocalinux.ui.config_manager.CONFIG_DIR", str(config_dir)),