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_prepare_aur_source.py

@@ -35,10 +35,10 @@ def sdist(
     extra: tuple[str, ...] = (),
 ) -> Path:
     """Write a small real sdist with independently controllable metadata/members."""
-    path = directory / f"vocalinux-{version}.tar.gz"
-    info = metadata if metadata is not None else f"Name: vocalinux\nVersion: {version}\n"
+    path = directory / f"voculinux-{version}.tar.gz"
+    info = metadata if metadata is not None else f"Name: voculinux\nVersion: {version}\n"
     with tarfile.open(path, "w:gz") as archive:
-        for name, text in [(f"vocalinux-{version}/PKG-INFO", info)] + [
+        for name, text in [(f"voculinux-{version}/PKG-INFO", info)] + [
             (name, "payload") for name in extra
         ]:
             member = tarfile.TarInfo(name)
@@ -208,11 +208,11 @@ def test_release_verification_cannot_be_silently_skipped(
     "metadata,extra",
     [
         ("Name: other\nVersion: 0.18.4\n", ()),
-        ("Name: vocalinux\nVersion: 0.18.3\n", ()),
-        ("Name: vocalinux\nVersion: 0.18.4\nVersion: 0.18.3\n", ()),
+        ("Name: voculinux\nVersion: 0.18.3\n", ()),
+        ("Name: voculinux\nVersion: 0.18.4\nVersion: 0.18.3\n", ()),
         (None, ("other-root/file",)),
-        (None, ("vocalinux-0.18.4/../unsafe",)),
-        (None, ("vocalinux-0.18.4/PKG-INFO",)),
+        (None, ("voculinux-0.18.4/../unsafe",)),
+        (None, ("voculinux-0.18.4/PKG-INFO",)),
     ],
 )
 def test_wrong_or_ambiguous_archive_metadata_is_rejected(