feat(sim): add variants/sim/ — real companion_radio firmware on native + Emscripten

New board variant compiling the unmodified MyMesh/UITask/DataStore app
logic against real mesh::Radio/MainBoard/RTCClock/RNG interfaces, for
running the actual firmware outside embedded hardware:

- Native (plain g++, platform = native): ASCII-art display over stdout,
  stdin-driven input, local-disk-backed DataStore/IdentityStore.
- Emscripten/WASM (variants/sim/build_wasm.sh, since PlatformIO's native
  platform force-overrides any CC/CXX toolchain override back to system
  clang++): canvas-backed display, IDBFS-backed persistence across page
  reloads, JS-callable input via sim_enqueue_key(), emscripten_set_main_loop.

Real rweather/Crypto (AES128/SHA256/Ed25519) vendored unmodified and
proven working on both targets. variants/sim/web/index.html is a bare
verification harness, not the polished website embed.

examples/companion_radio/DataStore.cpp

@@ -201,6 +201,8 @@ bool DataStore::formatFileSystem() {
   bool fs_success = ((fs::SPIFFSFS *)_fs)->format();
   esp_err_t nvs_err = nvs_flash_erase(); // no need to reinit, will be done by reboot
   return fs_success && (nvs_err == ESP_OK);
+#elif defined(SIM_PLATFORM)
+  return _fs->format();
 #else
   #error "need to implement format()"
 #endif