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/SimRTCClock.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <MeshCore.h>
+#include <ctime>
+
+// mesh::RTCClock backed by the real host wall clock (time(nullptr)), with
+// setCurrentTime() applying an offset so DataStore::restoreRTCTime()
+// (persisted last-known time) and the CLI's `time` command still work.
+class SimRTCClock : public mesh::RTCClock {
+  long _offset = 0;   // added to real wall-clock time()
+public:
+  uint32_t getCurrentTime() override {
+    return (uint32_t)((long)time(NULL) + _offset);
+  }
+  void setCurrentTime(uint32_t t) override {
+    _offset = (long)t - (long)time(NULL);
+  }
+};