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/rs232_port_owner.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "esp_err.h"
+
+typedef enum {
+    RS232_PORT_OWNER_NONE,
+    RS232_PORT_OWNER_PHASE0,
+    RS232_PORT_OWNER_SERVICE,
+    RS232_PORT_OWNER_FAULT,
+} rs232_port_owner_t;
+
+esp_err_t rs232_port_owner_init(void);
+esp_err_t rs232_port_claim(rs232_port_owner_t owner);
+esp_err_t rs232_port_release(rs232_port_owner_t owner);
+void rs232_port_mark_fault(rs232_port_owner_t previous_owner);
+rs232_port_owner_t rs232_port_get_owner(void);
+const char *rs232_port_owner_to_string(rs232_port_owner_t owner);