feat(slave_node): Implement uptime registers

- Add a callback for reading Modbus input registers.
- Implement logic to provide the system uptime in seconds, split across two 16-bit registers (UPTIME_SECONDS_LOW at 0x00F3 and UPTIME_SECONDS_HIGH at 0x00F4) as per documentation.
- Return 0 for unhandled registers to prevent "Invalid data" errors with certain Modbus masters.

software/apps/slave_node/src/main.c

@@ -68,11 +68,33 @@ static int holding_reg_wr(uint16_t addr, uint16_t reg)
 	return 0;
 }
 
+static int input_reg_rd(uint16_t addr, uint16_t *reg)
+{
+	uint32_t uptime_s = k_uptime_get_32() / 1000;
+
+	switch (addr) {
+	case 0x00F3:
+		*reg = (uint16_t)(uptime_s & 0xFFFF);
+		break;
+	case 0x00F4:
+		*reg = (uint16_t)(uptime_s >> 16);
+		break;
+	default:
+		*reg = 0;
+		break;
+	}
+
+	LOG_INF("Input register read, addr %u, value %u", addr, *reg);
+
+	return 0;
+}
+
 static struct modbus_user_callbacks mbs_cbs = {
 	.coil_rd = coil_rd,
 	.coil_wr = coil_wr,
 	.holding_reg_rd = holding_reg_rd,
 	.holding_reg_wr = holding_reg_wr,
+	.input_reg_rd = input_reg_rd,
 };
 
 const static struct modbus_iface_param server_param = {