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_resource_manager_ext.py

@@ -162,10 +162,10 @@ class TestResourceManagerCandidateSelection:
             (full_candidate / "sounds").mkdir(parents=True, exist_ok=True)
 
             for icon in [
-                "vocalinux",
-                "vocalinux-microphone",
-                "vocalinux-microphone-off",
-                "vocalinux-microphone-process",
+                "voculinux",
+                "voculinux-microphone",
+                "voculinux-microphone-off",
+                "voculinux-microphone-process",
             ]:
                 (full_candidate / "icons" / "scalable" / f"{icon}.svg").write_text("x")
 
@@ -183,7 +183,7 @@ class TestResourceManagerCandidateSelection:
         ResourceManager._resources_dir = None
         manager = ResourceManager()
 
-        missing = Path("/tmp/this-path-should-not-exist-vocalinux-resource-manager")
+        missing = Path("/tmp/this-path-should-not-exist-voculinux-resource-manager")
         candidate_score = getattr(manager, "_candidate_score")
         score = candidate_score(missing)
         assert score == -1