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

packaging/flatpak/index.html

@@ -27,7 +27,7 @@
     <h1>VocaHQ Flatpak remote</h1>
     <p>
       Self-hosted, signed OSTree repository for
-      <a href="https://vocalinux.com">Vocalinux</a> — published on every stable
+      <a href="https://voculinux.com">Voculinux</a> — published on every stable
       tagged release, so installs auto-update via <code>flatpak update</code>.
     </p>
     <pre><code>flatpak install __FLATPAKREF_URL__</code></pre>
@@ -38,11 +38,11 @@
     </p>
     <pre><code>flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
 flatpak remote-add --if-not-exists vocahq __FLATPAKREPO_URL__
-flatpak install vocahq com.vocalinux.Vocalinux</code></pre>
+flatpak install vocahq com.voculinux.Voculinux</code></pre>
     <p>
       The GNOME runtime still comes from Flathub — the
       <code>.flatpakref</code> declares <code>RuntimeRepo</code> so flatpak
-      finds it there. Vocalinux itself is not on Flathub.
+      finds it there. Voculinux itself is not on Flathub.
     </p>
     <p>
       <a href="https://github.com/VocaHQ/vocalinux">Source code</a> ·