Fix web OTA upload and isolate OTA sessions across firmware and goTool.

Split ESP-NOW into core/master/slave modules, block non-OTA UART traffic during updates, and hold the host serial port exclusively so dashboard polling cannot interleave with firmware uploads.

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

goTool/serial_link.go

@@ -11,14 +11,18 @@ import (
 // errUARTBusy is returned when the port is held for OTA (poller should not treat as unplug).
 var errUARTBusy = errors.New("uart busy (OTA in progress)")
 
+// errOTAInProgress is returned when a second OTA upload is attempted while one is running.
+var errOTAInProgress = errors.New("OTA upload already in progress")
+
 // 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
+	mu        sync.Mutex
+	sp        *serialPort
+	otaActive bool // UART held for firmware upload; poll/API must not interleave
 }
 
 func newManagedSerial(portName string, baud int) *managedSerial {
@@ -76,20 +80,17 @@ func (m *managedSerial) withPort(fn func(*serialPort) error) error {
 	return m.withPortLocked(false, fn)
 }
 
-// withPortPoll is like withPort but returns errUARTBusy instead of blocking during OTA.
+// withPortPoll is like withPort but returns errUARTBusy during OTA (no TryLock race).
 func (m *managedSerial) withPortPoll(fn func(*serialPort) error) error {
 	return m.withPortLocked(true, fn)
 }
 
-func (m *managedSerial) withPortLocked(try bool, fn func(*serialPort) error) error {
-	if try {
-		if !m.mu.TryLock() {
-			return errUARTBusy
-		}
-	} else {
-		m.mu.Lock()
-	}
+func (m *managedSerial) withPortLocked(poll bool, fn func(*serialPort) error) error {
+	m.mu.Lock()
 	defer m.mu.Unlock()
+	if m.otaActive {
+		return errUARTBusy
+	}
 
 	if m.sp == nil {
 		if err := m.openLocked(); err != nil {