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/SimLocationProvider.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <helpers/sensors/LocationProvider.h>
+
+// LocationProvider stub for the native sim build: no real GPS, always
+// reports "no fix". A settable lat/lon (JS-driven) is a Phase-2/3 concern.
+class SimLocationProvider : public LocationProvider {
+public:
+  long getLatitude() override { return 0; }
+  long getLongitude() override { return 0; }
+  long getAltitude() override { return 0; }
+  long satellitesCount() override { return 0; }
+  bool isValid() override { return false; }
+  long getTimestamp() override { return 0; }
+  void reset() override { }
+  void begin() override { }
+  void stop() override { }
+  void loop() override { }
+  bool isEnabled() override { return false; }
+};