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/simple_repeater/MyMesh.cpp

@@ -1022,6 +1022,8 @@ bool MyMesh::formatFileSystem() {
   return LittleFS.format();
 #elif defined(ESP32)
   return SPIFFS.format();
+#elif defined(SIM_PLATFORM)
+  return _fs->format();
 #else
 #error "need to implement file system erase"
   return false;
@@ -1181,6 +1183,8 @@ void MyMesh::saveIdentity(const mesh::LocalIdentity &new_id) {
   IdentityStore store(*_fs, "/identity");
 #elif defined(RP2040_PLATFORM)
   IdentityStore store(*_fs, "/identity");
+#elif defined(SIM_PLATFORM)
+  IdentityStore store(*_fs, "/identity");
 #else
 #error "need to define saveIdentity()"
 #endif