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

.github/workflows/snap-promote.yml

@@ -77,7 +77,7 @@ jobs:
         run: |
           set -euo pipefail
           VERSION="${TAG#v}"
-          /snap/bin/snapcraft status vocalinux | tee /tmp/snap-status.txt
+          /snap/bin/snapcraft status voculinux | tee /tmp/snap-status.txt
           # POSIX: mawk has no \S — use a literal non-space class.
           row=$(awk '/^[^ \t]/ { track = $1 }
                     track == "latest" && $1 == "candidate" { print $2, $3; exit }
@@ -110,7 +110,7 @@ jobs:
           row=$(awk '/^[^ \t]/ { track = $1 }
                     track == "latest" && $1 == "candidate" { print $2, $3; exit }
                     track == "latest" && $3 == "candidate" { print $4, $5; exit }' \
-                    <(/snap/bin/snapcraft status vocalinux))
+                    <(/snap/bin/snapcraft status voculinux))
           read -r seen_version revision <<<"$row"
           if [ "${revision:-}" != "$REVISION" ] || [ "$seen_version" != "$VERSION" ]; then
             echo "latest/candidate moved on (now version=${seen_version:-none} revision=${revision:-none}); aborting instead of promoting the stale revision" >&2
@@ -123,5 +123,5 @@ jobs:
           REVISION: ${{ steps.candidate.outputs.revision }}
         run: |
           set -euo pipefail
-          /snap/bin/snapcraft release vocalinux "$REVISION" latest/stable
-          /snap/bin/snapcraft status vocalinux
+          /snap/bin/snapcraft release voculinux "$REVISION" latest/stable
+          /snap/bin/snapcraft status voculinux