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.c

@@ -0,0 +1,89 @@
+#include "rs232_port_owner.h"
+
+#include "freertos/FreeRTOS.h"
+#include "freertos/semphr.h"
+
+static SemaphoreHandle_t s_owner_mutex;
+static rs232_port_owner_t s_owner = RS232_PORT_OWNER_NONE;
+
+esp_err_t rs232_port_owner_init(void)
+{
+    if (s_owner_mutex != NULL) {
+        return ESP_ERR_INVALID_STATE;
+    }
+    s_owner_mutex = xSemaphoreCreateMutex();
+    return s_owner_mutex != NULL ? ESP_OK : ESP_ERR_NO_MEM;
+}
+
+esp_err_t rs232_port_claim(rs232_port_owner_t owner)
+{
+    if (s_owner_mutex == NULL || owner == RS232_PORT_OWNER_NONE || owner == RS232_PORT_OWNER_FAULT) {
+        return ESP_ERR_INVALID_ARG;
+    }
+
+    xSemaphoreTake(s_owner_mutex, portMAX_DELAY);
+    esp_err_t result = ESP_ERR_INVALID_STATE;
+    if (s_owner == RS232_PORT_OWNER_NONE) {
+        s_owner = owner;
+        result = ESP_OK;
+    }
+    xSemaphoreGive(s_owner_mutex);
+    return result;
+}
+
+esp_err_t rs232_port_release(rs232_port_owner_t owner)
+{
+    if (s_owner_mutex == NULL) {
+        return ESP_ERR_INVALID_STATE;
+    }
+
+    xSemaphoreTake(s_owner_mutex, portMAX_DELAY);
+    esp_err_t result = ESP_ERR_INVALID_STATE;
+    if (s_owner == owner) {
+        s_owner = RS232_PORT_OWNER_NONE;
+        result = ESP_OK;
+    }
+    xSemaphoreGive(s_owner_mutex);
+    return result;
+}
+
+void rs232_port_mark_fault(rs232_port_owner_t previous_owner)
+{
+    if (s_owner_mutex == NULL) {
+        return;
+    }
+
+    xSemaphoreTake(s_owner_mutex, portMAX_DELAY);
+    if (s_owner == previous_owner) {
+        s_owner = RS232_PORT_OWNER_FAULT;
+    }
+    xSemaphoreGive(s_owner_mutex);
+}
+
+rs232_port_owner_t rs232_port_get_owner(void)
+{
+    if (s_owner_mutex == NULL) {
+        return RS232_PORT_OWNER_FAULT;
+    }
+
+    xSemaphoreTake(s_owner_mutex, portMAX_DELAY);
+    rs232_port_owner_t owner = s_owner;
+    xSemaphoreGive(s_owner_mutex);
+    return owner;
+}
+
+const char *rs232_port_owner_to_string(rs232_port_owner_t owner)
+{
+    switch (owner) {
+    case RS232_PORT_OWNER_NONE:
+        return "idle";
+    case RS232_PORT_OWNER_PHASE0:
+        return "Phase 0 diagnostics";
+    case RS232_PORT_OWNER_SERVICE:
+        return "serial service";
+    case RS232_PORT_OWNER_FAULT:
+        return "fault (reboot required)";
+    default:
+        return "unknown";
+    }
+}