feat(lib): Introduce adc_sensor library

Adds a new `adc_sensor` library to abstract reading analog values from ADC channels. The output of this library is currently simulated.

This library is now used by the `modbus_server` to read the motor current and the main supply voltage, replacing the previous implementation. This change improves modularity by centralizing ADC-related code into a dedicated module.

The build system has been updated to include the new library.

software/lib/adc_sensor/adc_sensor.c

@@ -0,0 +1,61 @@
+#include <zephyr/kernel.h>
+#include <zephyr/logging/log.h>
+#include <lib/adc_sensor.h>
+
+LOG_MODULE_REGISTER(adc_sensor, LOG_LEVEL_INF);
+
+// Simulated values
+#define SIMULATED_VOLTAGE_MV 12000
+#define SIMULATED_CURRENT_MA 45
+
+static bool initialized = false;
+
+int adc_sensor_init(void)
+{
+    if (initialized) {
+        return 0;
+    }
+
+#ifdef CONFIG_ADC_SENSOR_SIMULATED
+    LOG_INF("ADC sensor initialized (simulated mode)");
+    LOG_INF("Simulated values: %dmV, %dmA", SIMULATED_VOLTAGE_MV, SIMULATED_CURRENT_MA);
+#else
+    // TODO: Initialize real ADC hardware
+    LOG_INF("ADC sensor initialized (real ADC mode - not yet implemented)");
+#endif
+
+    initialized = true;
+    return 0;
+}
+
+uint16_t adc_sensor_get_voltage_mv(void)
+{
+    if (!initialized) {
+        LOG_WRN("ADC sensor not initialized, calling adc_sensor_init()");
+        adc_sensor_init();
+    }
+
+#ifdef CONFIG_ADC_SENSOR_SIMULATED
+    return SIMULATED_VOLTAGE_MV;
+#else
+    // TODO: Read real ADC value for voltage
+    // For now return simulated value
+    return SIMULATED_VOLTAGE_MV;
+#endif
+}
+
+uint16_t adc_sensor_get_current_ma(void)
+{
+    if (!initialized) {
+        LOG_WRN("ADC sensor not initialized, calling adc_sensor_init()");
+        adc_sensor_init();
+    }
+
+#ifdef CONFIG_ADC_SENSOR_SIMULATED
+    return SIMULATED_CURRENT_MA;
+#else
+    // TODO: Read real ADC value for current
+    // For now return simulated value
+    return SIMULATED_CURRENT_MA;
+#endif
+}