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/MyMesh.cpp

@@ -2845,7 +2845,12 @@ void MyMesh::handleCmdFrame(size_t len) {
     bool success = _store->formatFileSystem();
     if (success) {
       writeOKFrame();
+#ifdef SIM_PLATFORM
+      // Skip the pre-reboot UX pause -- board.reboot() just exits the
+      // process in the sim (see SimMainBoard::reboot()).
+#else
       delay(1000);
+#endif
       board.reboot();  // doesn't return
     } else {
       writeErrFrame(ERR_CODE_FILE_IO_ERROR);