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/Stream.h

@@ -0,0 +1,33 @@
+#pragma once
+
+// Minimal native stand-in for Arduino's Stream class. src/Utils.h includes
+// <Stream.h> directly (mesh::Utils::printHex(Stream&, ...)), and
+// src/Identity.cpp's readFrom/writeTo/printTo take a Stream& -- both real
+// hardware File objects (fs::File, Adafruit_LittleFS's File) and our own
+// SimFile (variants/sim/SimFS.h) derive from this, exactly like on real
+// boards.
+
+#include "Print.h"
+
+class Stream : public Print {
+public:
+  virtual ~Stream() = default;
+
+  virtual int available() { return 0; }
+  virtual int availableForWrite() { return 0; }
+  virtual int read() { return -1; }
+  virtual int peek() { return -1; }
+
+  virtual size_t readBytes(char *buffer, size_t length) {
+    size_t i = 0;
+    while (i < length) {
+      int c = read();
+      if (c < 0) break;
+      buffer[i++] = (char)c;
+    }
+    return i;
+  }
+  size_t readBytes(uint8_t *buffer, size_t length) {
+    return readBytes((char *)buffer, length);
+  }
+};