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>

examples/companion_radio/MyMesh.cpp

@@ -3294,6 +3294,29 @@ bool MyMesh::advert() {
   }
 }
 
+#ifdef SIM_PLATFORM
+bool MyMesh::advertFlood() {
+  // Mirrors the CMD_SEND_SELF_ADVERT handler's flood=1 branch above
+  // (createSelfAdvert() + sendFloodScoped() with the default transport
+  // scope key) -- real protocol logic, just reached from a test-only entry
+  // point instead of a parsed serial command frame.
+  mesh::Packet* pkt;
+  if (_prefs.advert_loc_policy == ADVERT_LOC_NONE) {
+    pkt = createSelfAdvert(_prefs.node_name);
+  } else {
+    pkt = createSelfAdvert(_prefs.node_name, sensors.node_lat, sensors.node_lon);
+  }
+  if (pkt) {
+    TransportKey default_scope;
+    memcpy(&default_scope.key, _prefs.default_scope_key, sizeof(default_scope.key));
+    sendFloodScoped(default_scope, pkt, 0);
+    return true;
+  } else {
+    return false;
+  }
+}
+#endif
+
 // To check if there is pending work
 bool MyMesh::hasPendingWork() const {
   return _mgr->getOutboundTotal() > 0 || dirty_contacts_expiry != 0;