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_flatpak_packaging.py

@@ -47,7 +47,7 @@ PYPROJECT = REPO_ROOT / "pyproject.toml"
 EXPORT = REPO_ROOT / "requirements" / "vad.txt"
 FLATPAK_DIR = REPO_ROOT / "packaging" / "flatpak"
 DEPS_YAML = FLATPAK_DIR / "python3-dependencies.yaml"
-MANIFEST = FLATPAK_DIR / "com.vocalinux.Vocalinux.yml"
+MANIFEST = FLATPAK_DIR / "com.voculinux.Voculinux.yml"
 WORKFLOW = REPO_ROOT / ".github" / "workflows" / "flatpak.yml"
 
 #: Kept in step with scripts/sync_flatpak_deps.py, which resolves the same