Add Phase 1 UART service foundation

Add versioned NVS-backed configuration, buffered UART1 I/O, modem
monitoring, counters, and serial console controls. Coordinate UART1
ownership with Phase 0 diagnostics and document loopback verification.

src/serial_console.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#include "esp_err.h"
+
+/* Register Phase 1 serial configuration and UART-service console commands. */
+esp_err_t serial_console_register_commands(void);