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_python_version_policy.py

@@ -14,7 +14,7 @@ REPO_ROOT = Path(__file__).resolve().parents[1]
 PYPROJECT = REPO_ROOT / "pyproject.toml"
 INSTALL_SH = REPO_ROOT / "install.sh"
 PIPELINE = REPO_ROOT / ".github" / "workflows" / "unified-pipeline.yml"
-PKGBUILD = REPO_ROOT / "packaging" / "aur" / "vocalinux" / "PKGBUILD"
+PKGBUILD = REPO_ROOT / "packaging" / "aur" / "voculinux" / "PKGBUILD"
 
 # Everything user-facing that could state the floor. Scanned as trees rather than
 # a fixed list: docs and the website drifted to "3.9+" for a full release cycle