Add goTool clients subcommand for CLIENT_INFO query.

Refactor into version/clients subcommands with shared serial framing
to list registered slaves from the master over UART.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/serialport.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"time"
+
+	"go.bug.st/serial"
+	uartframe "powerpod/gotool/uart"
+)
+
+const readTimeout = 3 * time.Second
+
+type serialPort struct {
+	port serial.Port
+}
+
+func openSerial(portName string, baud int) (*serialPort, error) {
+	mode := &serial.Mode{
+		BaudRate: baud,
+		DataBits: 8,
+		Parity:   serial.NoParity,
+		StopBits: serial.OneStopBit,
+	}
+
+	port, err := serial.Open(portName, mode)
+	if err != nil {
+		return nil, err
+	}
+	if err := port.SetReadTimeout(readTimeout); err != nil {
+		port.Close()
+		return nil, err
+	}
+	return &serialPort{port: port}, nil
+}
+
+func (s *serialPort) Close() error {
+	return s.port.Close()
+}
+
+func (s *serialPort) exchange(cmdID byte, cmdName string) ([]byte, error) {
+	frame, err := uartframe.EncodeFrame([]byte{cmdID})
+	if err != nil {
+		return nil, fmt.Errorf("encode frame: %w", err)
+	}
+
+	log.Printf("sending %s command (%d bytes): % x", cmdName, len(frame), frame)
+	if _, err := s.port.Write(frame); err != nil {
+		return nil, fmt.Errorf("write: %w", err)
+	}
+
+	payload, err := uartframe.ReadFrame(s.port, nil)
+	if err != nil {
+		return nil, fmt.Errorf("read response: %w", err)
+	}
+
+	log.Printf("response payload (%d bytes): % x", len(payload), payload)
+	if len(payload) == 0 {
+		return nil, fmt.Errorf("empty response payload")
+	}
+	if payload[0] != cmdID {
+		return nil, fmt.Errorf("unexpected command id 0x%02x (want 0x%02x)", payload[0], cmdID)
+	}
+	return payload, nil
+}