fix: a new playback raced the previous one for the audio device

Reported precisely: press play again and it stops the current message, but you
then have to wait the whole length of the message before it will play — otherwise
the radio is keyed and the PTT released at once.

Play only SIGNALLED the previous playback to stop and started a new one straight
away. The old goroutine still held the WASAPI render client for a moment, so the
new client could not start; it returned immediately and the PTT tail was all that
was left. The log now shows both halves of that: a play line, then a release
120 ms later.

Stopping now waits for the goroutine to have returned — the point of stopping
before starting again — bounded at 1.5 s so a wedged device cannot freeze the
operator's click, and saying so in the log if it ever comes to that.

internal/audio/manager.go

@@ -5,6 +5,7 @@ package audio
 import (
 	"fmt"
 	"sync"
+	"time"
 )
 
 // Manager owns the DVK record/playback lifecycle: at most one recording and
@@ -15,6 +16,10 @@ type Manager struct {
 	recStop  chan struct{}
 	recDone  chan recResult
 	playStop chan struct{}
+	// playDone closes when the playback goroutine has returned and the audio
+	// device is free again. Without it, the next Play raced the old one for the
+	// device and lost.
+	playDone chan struct{}
 	monStop  chan struct{} // RX monitor passthrough (capture → render)
 	monRing  *pcmRing      // live audio hand-off, also fed by the network stream
 	txStop   chan struct{} // TX audio passthrough (mic → rig)
@@ -125,10 +130,20 @@ func (m *Manager) Play(deviceID, path string, gainPct int) error {
 			pcm[i], pcm[i+1] = byte(uint16(v)), byte(uint16(v)>>8)
 		}
 	}
+	// Waits for any previous playback to have RELEASED THE DEVICE.
+	//
+	// It used to only signal the old one to stop and start a new one at once.
+	// The old goroutine still held the WASAPI render client for a moment, so the
+	// new client could not start: it returned immediately, the PTT was keyed and
+	// released a tenth of a second later, and nothing came out. On the air that
+	// looked like "press play again and you must wait the whole length of the
+	// message before it will play at all".
 	m.StopPlayback()
 	stop := make(chan struct{})
+	done := make(chan struct{})
 	m.mu.Lock()
 	m.playStop = stop
+	m.playDone = done
 	m.mu.Unlock()
 	go func() {
 		// The error was discarded. A device that refuses to start returns here
@@ -142,7 +157,11 @@ func (m *Manager) Play(deviceID, path string, gainPct int) error {
 		if m.playStop == stop {
 			m.playStop = nil
 		}
+		if m.playDone == done {
+			m.playDone = nil
+		}
 		m.mu.Unlock()
+		close(done) // the device is free from here
 		m.notify()
 	}()
 	m.notify()
@@ -153,12 +172,24 @@ func (m *Manager) Play(deviceID, path string, gainPct int) error {
 func (m *Manager) StopPlayback() {
 	m.mu.Lock()
 	stop := m.playStop
+	done := m.playDone
 	m.playStop = nil
 	m.mu.Unlock()
-	if stop != nil {
-		close(stop)
-		m.notify()
+	if stop == nil {
+		return
 	}
+	close(stop)
+	// Wait for the goroutine to release the device — that is the whole point of
+	// stopping before starting again. Bounded: a wedged WASAPI call must not
+	// freeze the caller, which here is the operator clicking a button.
+	if done != nil {
+		select {
+		case <-done:
+		case <-time.After(1500 * time.Millisecond):
+			LogSink("audio: previous playback did not release the device within 1.5 s")
+		}
+	}
+	m.notify()
 }
 
 // ---- RX audio monitor (Phase 2: USB codec passthrough) --------------------