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.

variants/sim/thirdparty/arduinojson/ArduinoJson.hpp

@@ -0,0 +1,65 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2026, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#if __cplusplus < 201103L && (!defined(_MSC_VER) || _MSC_VER < 1910)
+#  error ArduinoJson requires C++11 or newer. Configure your compiler for C++11 or downgrade ArduinoJson to 6.20.
+#endif
+
+#include "ArduinoJson/Configuration.hpp"
+
+// Include Arduino.h before stdlib.h to avoid conflict with atexit()
+// https://github.com/bblanchon/ArduinoJson/pull/1693#issuecomment-1001060240
+#if ARDUINOJSON_ENABLE_ARDUINO_STRING || ARDUINOJSON_ENABLE_ARDUINO_STREAM || \
+    ARDUINOJSON_ENABLE_ARDUINO_PRINT ||                                       \
+    (ARDUINOJSON_ENABLE_PROGMEM && defined(ARDUINO))
+#  include <Arduino.h>
+#endif
+
+#if !ARDUINOJSON_DEBUG
+#  ifdef __clang__
+#    pragma clang system_header
+#  elif defined __GNUC__
+#    pragma GCC system_header
+#  endif
+#endif
+
+// Remove true and false macros defined by some cores, such as Arduino Due's
+// See issues #2181 and arduino/ArduinoCore-sam#50
+#ifdef true
+#  undef true
+#endif
+#ifdef false
+#  undef false
+#endif
+
+#include "ArduinoJson/Array/JsonArray.hpp"
+#include "ArduinoJson/Object/JsonObject.hpp"
+#include "ArduinoJson/Variant/JsonVariantConst.hpp"
+
+#include "ArduinoJson/Document/JsonDocument.hpp"
+
+#include "ArduinoJson/Array/ArrayImpl.hpp"
+#include "ArduinoJson/Array/ElementProxy.hpp"
+#include "ArduinoJson/Array/Utilities.hpp"
+#include "ArduinoJson/Collection/CollectionImpl.hpp"
+#include "ArduinoJson/Memory/ResourceManagerImpl.hpp"
+#include "ArduinoJson/Object/MemberProxy.hpp"
+#include "ArduinoJson/Object/ObjectImpl.hpp"
+#include "ArduinoJson/Variant/ConverterImpl.hpp"
+#include "ArduinoJson/Variant/JsonVariantCopier.hpp"
+#include "ArduinoJson/Variant/VariantCompare.hpp"
+#include "ArduinoJson/Variant/VariantImpl.hpp"
+#include "ArduinoJson/Variant/VariantRefBaseImpl.hpp"
+
+#include "ArduinoJson/Json/JsonDeserializer.hpp"
+#include "ArduinoJson/Json/JsonSerializer.hpp"
+#include "ArduinoJson/Json/PrettyJsonSerializer.hpp"
+#include "ArduinoJson/MsgPack/MsgPackBinary.hpp"
+#include "ArduinoJson/MsgPack/MsgPackDeserializer.hpp"
+#include "ArduinoJson/MsgPack/MsgPackExtension.hpp"
+#include "ArduinoJson/MsgPack/MsgPackSerializer.hpp"
+
+#include "ArduinoJson/compatibility.hpp"