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/PERF_BASELINE_PROTOCOL.md

@@ -19,7 +19,7 @@ no CUDA toolkit installed (Vulkan then CPU backend order).
   `small.en` (floor, by language).
 - Isolated XDG directories so the pass never touches a live install:
   point `XDG_CONFIG_HOME`, `XDG_DATA_HOME`, `XDG_STATE_HOME` at scratch
-  roots and link the verified models under `<data>/vocalinux/models/`.
+  roots and link the verified models under `<data>/voculinux/models/`.
 - Seed `config.json` from the app's own `DEFAULT_CONFIG`: engine
   `whisper_cpp`, the rung under test, keep-alive enabled with the
   minimum 60 s timeout, `first_run` false, update checks off.
@@ -36,8 +36,8 @@ with the installed build's interpreter.
    `reason=idle_keepalive`). RSS does not reflect the unload (freed
    native pages stay mapped); VRAM-used is the state signal.
 2. **Wake latency, toggle to capture start.** Drive starts over D-Bus
-   (`busctl --user call com.vocalinux.Vocalinux
-   /com/vocalinux/Vocalinux com.vocalinux.Vocalinux Toggle`; the
+   (`busctl --user call com.voculinux.Voculinux
+   /com/voculinux/Voculinux com.voculinux.Voculinux Toggle`; the
    `--toggle` client also works but spawns the full app stack).
    Parse repeated runs with `parse-log --log`: each `Toggle requested
    in state IDLE` pairs with the next `Opened capture stream`. Report