Add Phase 0 RS-232 characterization harness and test firmware with cli

src/rs232_hw_test.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "esp_err.h"
+
+/* Configure all MAX3243 logic-side signals in their safe static-test state. */
+esp_err_t rs232_hw_test_init(void);
+
+/* Register the Phase 0 hardware-characterization commands with esp_console. */
+esp_err_t rs232_hw_test_register_console_commands(void);