Add wstring to test shims

test/shims/WString.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <cstddef>
+#include <cstring>
+
+// Minimal host-side stand-in for the Arduino framework's WString/String.
+// Only provides the surface that header-only inline overloads in this repo
+// touch (c_str/length). Host tests never construct a String.
+class String {
+ public:
+  String() = default;
+  String(const char* s) : _buf(s ? s : "") {}
+
+  const char* c_str() const { return _buf; }
+  size_t length() const { return std::strlen(_buf); }
+
+ private:
+  const char* _buf = "";
+};