add ESP32 build system with xtensa-esp-elf toolchain

build/esp32/main_esp32.cpp

@@ -0,0 +1,8 @@
+// ESP32 entry point for Spider Runtime
+// This replaces SpiderRuntime.cpp for microcontroller builds
+#include <spider/SpiderRuntime.hpp>
+
+int main() {
+    // TODO: initialize Spider runtime for ESP32
+    return 0;
+}