Harden observer preferences and portable controls

test/mocks/Arduino.h

@@ -2,6 +2,7 @@
 
 #include <cstdint>
 #include <cmath>
+#include <cstdio>
 #include "Stream.h"
 
 inline uint32_t g_mock_millis = 0;
@@ -15,3 +16,12 @@ inline uint32_t millis() {
 inline void delay(uint32_t ms) {
   g_mock_millis += ms;
 }
+
+inline char* ltoa(long value, char* dest, int base) {
+  if (base == 10) {
+    std::snprintf(dest, 16, "%ld", value);
+  } else {
+    dest[0] = '\0';
+  }
+  return dest;
+}