fix: make Pluto live LO retune fail loudly and avoid redundant channel lookups

Cache altvoltage1 during Configure(), clear it in cleanup(), and use the cached channel for Tune() instead of calling findChannel on every retune. Tune() now checks the direct iio_channel_attr_write_longlong return code and reports LO write failures back to the caller with the failing frequency and libiio error code.

internal/platform/plutosdr/pluto_windows.go

@@ -110,6 +110,7 @@ type PlutoDriver struct {
 	phyDev   uintptr // iio_device* (ad9361-phy)
 	chanI    uintptr // iio_channel* TX I
 	chanQ    uintptr // iio_channel* TX Q
+	chanLO   uintptr // iio_channel* TX LO (altvoltage1), cached for Tune()
 	buf      uintptr // iio_buffer*
 	bufSize  int     // samples per buffer push
 
@@ -204,6 +205,7 @@ func (d *PlutoDriver) Configure(_ context.Context, cfg platform.SoapyConfig) err
 
 	// TX LO frequency
 	phyChanLO := d.findChannel(phyDev, "altvoltage1", true) // TX LO
+	d.chanLO = phyChanLO // cache for Tune()
 	if phyChanLO != 0 {
 		freqHz := int64(cfg.CenterFreqHz)
 		if freqHz <= 0 {
@@ -371,14 +373,21 @@ func (d *PlutoDriver) Flush(_ context.Context) error { return nil }
 func (d *PlutoDriver) Tune(_ context.Context, freqHz float64) error {
 	d.mu.Lock()
 	defer d.mu.Unlock()
-	if d.phyDev == 0 {
-		return fmt.Errorf("pluto: not configured")
+	if !d.configured || d.chanLO == 0 {
+		return fmt.Errorf("pluto: not configured or LO channel not available")
+	}
+	if d.lib.pChannelAttrWriteLongLong == nil {
+		return fmt.Errorf("pluto: iio_channel_attr_write_longlong not loaded")
 	}
-	phyChanLO := d.findChannel(d.phyDev, "altvoltage1", true)
-	if phyChanLO == 0 {
-		return fmt.Errorf("pluto: TX LO channel not found")
+	cAttr, _ := syscall.BytePtrFromString("frequency")
+	ret, _, _ := d.lib.pChannelAttrWriteLongLong.Call(
+		d.chanLO,
+		uintptr(unsafe.Pointer(cAttr)),
+		uintptr(int64(freqHz)),
+	)
+	if int32(ret) < 0 {
+		return fmt.Errorf("pluto: LO tune to %.0f Hz failed (iio rc=%d)", freqHz, int32(ret))
 	}
-	d.writeChanAttrLL(phyChanLO, "frequency", int64(freqHz))
 	return nil
 }
 
@@ -420,6 +429,7 @@ func (d *PlutoDriver) cleanup() {
 		d.disableChannel(d.chanQ)
 		d.chanQ = 0
 	}
+	d.chanLO = 0 // config-only channel, no disable needed
 	if d.ctx != 0 && d.lib.pDestroyCtx != nil {
 		d.lib.pDestroyCtx.Call(d.ctx)
 		d.ctx = 0