feat(sim): port examples/simple_room_server to variants/sim/

Mirrors the simple_repeater sim port exactly: headless (no DISPLAY_CLASS,
UITask.cpp excluded from the build), new sim_simple_room_server native env
plus build_wasm_room_server.sh, own SimFS root ./sim_data_room so its
identity storage can't collide with the companion or repeater instances on
the same page/cwd.

Verified beyond "it compiles": ran the native binary and confirmed a real
_main.id identity file gets persisted through the actual SimFS/IdentityStore
path, same as the other two sim targets.

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

examples/simple_room_server/MyMesh.cpp

@@ -787,6 +787,8 @@ 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;
@@ -852,6 +854,8 @@ 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