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

docs/README.md

@@ -1,6 +1,6 @@
 # Documentation
 
-User and maintainer documentation for Vocalinux.
+User and maintainer documentation for Voculinux.
 
 | Document | Audience | Description |
 |----------|----------|-------------|
@@ -32,4 +32,4 @@ Project root:
 
 Website product and design notes live under [`web/`](../web/) (`PRODUCT.md`, `DESIGN.md`, `README.md`). Flatpak packaging: [`packaging/flatpak/README.md`](../packaging/flatpak/README.md).
 
-Website: [vocalinux.com](https://vocalinux.com)
+Website: [voculinux.com](https://voculinux.com)