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/target.cpp

@@ -0,0 +1,28 @@
+#include <Arduino.h>
+#include "target.h"
+#include "SimRNG.h"
+
+// Global stdout-backed Serial object, declared extern in Arduino.h.
+SimSerialClass Serial;
+
+SimMainBoard board;
+SimRadio radio_driver;
+SimRTCClock rtc_clock;
+SensorManager sensors;   // base class: no real sensors in Phase 1
+
+#ifdef DISPLAY_CLASS
+DISPLAY_CLASS display;
+#endif
+
+bool radio_init() {
+  // No real radio hardware to initialise -- always succeeds (see
+  // variants/sim/SimRadio.h; Phase 3 of the sim plan is where two SimRadio
+  // instances actually exchange bytes through a shared in-memory "ether").
+  return true;
+}
+
+mesh::LocalIdentity radio_new_identity() {
+  static SimRNG rng;
+  rng.begin();
+  return mesh::LocalIdentity(&rng);
+}