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.

src/helpers/IdentityStore.h

@@ -8,6 +8,11 @@
   #define FILESYSTEM  Adafruit_LittleFS
 
   using namespace Adafruit_LittleFS_Namespace;
+#elif defined(SIM_PLATFORM)
+  #include <SimFS.h>
+  #define FILESYSTEM  SimFS
+  // SimFS.h already typedefs File itself (matches the ESP32/NRF52 pattern of
+  // a bare, unqualified `File` symbol coming from the platform's own core).
 #endif
 #include <Identity.h>