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/CMakeLists.txt

@@ -3,6 +3,10 @@ idf_component_register(
         "main.c"
         "status_led.c"
         "rs232_hw_test.c"
+        "rs232_port_owner.c"
+        "serial_config.c"
+        "serial_service.c"
+        "serial_console.c"
     INCLUDE_DIRS "."
     REQUIRES
         console
@@ -12,4 +16,5 @@ idf_component_register(
         esp_timer
         freertos
         led_strip
+        nvs_flash
 )