feat(sim): two-device messaging + repeater relay over a JS ether

Ports examples/simple_repeater to variants/sim/ (new sim_simple_repeater
native env + build_wasm_repeater.sh) and adds a JS "ether"
(variants/sim/web/mesh.html) that bridges two real companion_radio WASM
instances through a real simple_repeater instance in a strict A<->R<->B
topology (no direct A-B link), proving genuine relay routing rather than
a shortcut.

Also fixes multi-instance issues Phase 2's single-instance design never
surfaced: SimDisplayDriver's canvas context/id caching was keyed on a
single global instead of per-instance, and both wasm builds were missing
_malloc/_free/HEAPU8 runtime exports needed for the ether to poke bytes
into an instance's memory.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

variants/sim/SimLocationProvider.h

@@ -1,20 +1,77 @@
 #pragma once
 
 #include <helpers/sensors/LocationProvider.h>
+#include <helpers/SensorManager.h>
+#include <ctime>
 
-// 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.
+// LocationProvider stub for the sim build. Phase 1/2: no real GPS, always
+// reports "no fix" (default-constructed state below reproduces that
+// exactly). Phase 3: JS-settable per instance via sim_location_set(),
+// declared at the bottom of this file.
+//
+// Note on how this actually reaches the advertised location: companion_radio
+// itself never calls a LocationProvider through this class -- it reads
+// `sensors.node_lat`/`node_lon` directly (see e.g.
+// examples/companion_radio/MyMesh.cpp's createSelfAdvert() call sites), and
+// `sensors` (declared in target.h) is a plain base `SensorManager`, not a
+// subclass wired to any LocationProvider -- there was never a real GPS
+// object plugged in here to begin with (see SensorManager.h: "modify
+// node_lat/node_lon directly, if you want to affect Advert location"). So
+// sim_location_set() below writes BOTH this class's own state (satisfying
+// the LocationProvider interface faithfully, in case future sim code wants
+// a real one) AND `sensors.node_lat/node_lon` directly (the field
+// companion_radio's advert path actually reads) -- belt and suspenders,
+// same underlying values either way.
 class SimLocationProvider : public LocationProvider {
+  long _lat_e6 = 0, _lon_e6 = 0, _alt_mm = 0;
+  bool _valid = false;
+
 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 { }
+  long getLatitude() override { return _lat_e6; }
+  long getLongitude() override { return _lon_e6; }
+  long getAltitude() override { return _alt_mm; }
+  long satellitesCount() override { return _valid ? 8 : 0; }
+  bool isValid() override { return _valid; }
+  long getTimestamp() override { return _valid ? (long)time(NULL) : 0; }
+  void reset() override { _valid = false; }
   void begin() override { }
   void stop() override { }
   void loop() override { }
-  bool isEnabled() override { return false; }
+  bool isEnabled() override { return _valid; }
+  void sendSentence(const char* sentence) override { }
+
+  void set(float lat_deg, float lon_deg, float alt_m = 0.0f) {
+    _lat_e6 = (long)(lat_deg * 1000000.0f);
+    _lon_e6 = (long)(lon_deg * 1000000.0f);
+    _alt_mm = (long)(alt_m * 1000.0f);
+    _valid = true;
+  }
 };
+
+// Single instance, mirroring the pattern of every other Sim* global
+// (board/radio_driver/rtc_clock/sensors) declared in target.h/target.cpp --
+// this one isn't wired into target.h/.cpp itself since nothing on the
+// SIM_PLATFORM boot path currently constructs a LocationProvider at all
+// (see the class comment above), so it lives here instead as a
+// self-contained, opt-in addition.
+extern SensorManager sensors;   // defined in variants/sim/target.cpp
+inline SimLocationProvider& sim_location_provider() {
+  static SimLocationProvider instance;
+  return instance;
+}
+
+inline void sim_location_apply(float lat_deg, float lon_deg, float alt_m = 0.0f) {
+  sim_location_provider().set(lat_deg, lon_deg, alt_m);
+  sensors.node_lat = lat_deg;
+  sensors.node_lon = lon_deg;
+  sensors.node_altitude = alt_m;
+}
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+// C linkage (no default args, unlike sim_location_apply() above) so JS's
+// ccall('sim_location_set', ...) can find it by its exact, unmangled name.
+extern "C" inline EMSCRIPTEN_KEEPALIVE void sim_location_set(float lat_deg, float lon_deg, float alt_m) {
+  sim_location_apply(lat_deg, lon_deg, alt_m);
+}
+#endif