Add goTool web dashboard with UART auto-reconnect.

Serve polls the master over UART and pushes live state via WebSocket;
reopens the serial port when the device is unplugged and comes back.

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

goTool/serial_link.go

@@ -0,0 +1,122 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"sync"
+	"time"
+)
+
+// managedSerial keeps the UART open and reconnects after I/O failures or unplug.
+type managedSerial struct {
+	portName string
+	baud     int
+	quiet    bool
+
+	mu sync.Mutex
+	sp *serialPort
+}
+
+func newManagedSerial(portName string, baud int) *managedSerial {
+	return &managedSerial{
+		portName: portName,
+		baud:     baud,
+	}
+}
+
+func (m *managedSerial) Close() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.closeLocked()
+}
+
+func (m *managedSerial) IsConnected() bool {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.sp != nil
+}
+
+func (m *managedSerial) openLocked() error {
+	sp, err := openSerial(m.portName, m.baud)
+	if err != nil {
+		return err
+	}
+	sp.quiet = m.quiet
+	m.sp = sp
+	if !m.quiet {
+		log.Printf("UART %s connected (%d baud)", m.portName, m.baud)
+	}
+	return nil
+}
+
+func (m *managedSerial) closeLocked() error {
+	if m.sp == nil {
+		return nil
+	}
+	err := m.sp.port.Close()
+	m.sp = nil
+	return err
+}
+
+func (m *managedSerial) invalidateLocked(reason error) {
+	if m.sp == nil {
+		return
+	}
+	if !m.quiet {
+		log.Printf("UART %s disconnected: %v", m.portName, reason)
+	}
+	_ = m.closeLocked()
+}
+
+func (m *managedSerial) withPort(fn func(*serialPort) error) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.sp == nil {
+		if err := m.openLocked(); err != nil {
+			return fmt.Errorf("%s: %w", m.portName, err)
+		}
+	}
+
+	err := fn(m.sp)
+	if err != nil {
+		m.invalidateLocked(err)
+	}
+	return err
+}
+
+func (m *managedSerial) exchangePayload(payload []byte, cmdName string) ([]byte, error) {
+	var resp []byte
+	err := m.withPort(func(sp *serialPort) error {
+		var e error
+		resp, e = sp.exchangePayloadLocked(payload, cmdName)
+		return e
+	})
+	return resp, err
+}
+
+func (m *managedSerial) exchange(cmdID byte, cmdName string) ([]byte, error) {
+	var resp []byte
+	err := m.withPort(func(sp *serialPort) error {
+		var e error
+		resp, e = sp.exchangeLocked(cmdID, cmdName)
+		return e
+	})
+	return resp, err
+}
+
+func disconnectedState(portName string, err error) DashboardState {
+	msg := "UART disconnected"
+	if err != nil {
+		msg = err.Error()
+	}
+	return DashboardState{
+		UpdatedAt:     time.Now().Format(time.RFC3339),
+		SerialPort:    portName,
+		UARTConnected: false,
+		SerialOK:      false,
+		SerialError:   msg,
+		Master:        MasterView{OK: false, Error: msg},
+		Clients:       []ClientView{},
+	}
+}