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/arduino/RTClib.h

@@ -0,0 +1,36 @@
+#pragma once
+// Minimal native stand-in for Adafruit's RTClib.h. Real hardware boards pull
+// this in transitively for helpers/AutoDiscoverRTCClock.h (not used by the
+// sim build -- SimRTCClock.h reads the host wall clock directly) and for
+// DateTime, which src/helpers/CommonCLI.cpp's "clock"/"clock sync"/"time"
+// CLI commands genuinely construct and call hour()/minute()/day()/month()/
+// year() on. Nothing in companion_radio (root files or ui-new/) uses
+// DateTime, but CommonCLI.cpp (built for every board, including the sim) does.
+//
+// Implemented via the real C library's gmtime_r() (UTC calendar breakdown)
+// rather than hand-rolling RTClib's own unixtime<->y/m/d/h/m/s conversion --
+// same result, zero risk of a transcription bug in that math.
+#include <ctime>
+#include <cstdint>
+
+class DateTime {
+  uint32_t _unixtime;
+public:
+  DateTime(uint32_t t = 0) : _unixtime(t) { }
+
+  uint16_t year() const   { return 1900 + tmFields().tm_year; }
+  uint8_t  month() const  { return (uint8_t)(tmFields().tm_mon + 1); }
+  uint8_t  day() const    { return (uint8_t)tmFields().tm_mday; }
+  uint8_t  hour() const   { return (uint8_t)tmFields().tm_hour; }
+  uint8_t  minute() const { return (uint8_t)tmFields().tm_min; }
+  uint8_t  second() const { return (uint8_t)tmFields().tm_sec; }
+  uint32_t unixtime() const { return _unixtime; }
+
+private:
+  struct tm tmFields() const {
+    time_t t = (time_t)_unixtime;
+    struct tm out;
+    gmtime_r(&t, &out);
+    return out;
+  }
+};