fix: write off an audio source that never delivers anything

The operator said it from the start: in CW there is no sidetone, only reception.
My previous fix did not cover it — it wrote off a source that STOPPED, but a
source that never starts has no last-delivery time, so it stayed forever "not
yet dry" and held the mixer at min(A,B)=0. Three empty recordings while the
radio audio streamed perfectly.

A source with no delivery is now measured from when capture started. Tested both
ways: nothing is written off in the first moments, and a source that has never
spoken is written off once the grace period has passed.

internal/audio/recorder.go

@@ -57,6 +57,9 @@ type Recorder struct {
 	// produces anything is not an error anywhere — the capture call just sits
 	// there — so the only way to notice is to watch the clock.
 	lastA, lastB time.Time
+	// startedAt is when capture began — the reference for a source that has not
+	// delivered anything at all yet.
+	startedAt time.Time
 	// deadB (deadA) latches once a source has been declared silent, so the
 	// warning is logged once rather than 25 times a second.
 	deadA, deadB bool
@@ -158,7 +161,9 @@ func (r *Recorder) Start(fromDev, micDev string, prerollSec int) error {
 	r.twoSrc = micDev != "" && micDev != fromDev
 	r.stopCh = make(chan struct{})
 	r.running = true
+	r.startedAt = time.Now()
 	r.ring, r.acc, r.active, r.paused, r.bufA, r.bufB = nil, nil, false, false, nil, nil
+	r.lastA, r.lastB, r.deadA, r.deadB = time.Time{}, time.Time{}, false, false
 	stop := r.stopCh
 	twoSrc := r.twoSrc
 	r.mu.Unlock()
@@ -265,10 +270,23 @@ func (r *Recorder) mixTick() {
 		// path is not running — and the operator got "recording was empty" after
 		// a whole QSO. Half a recording is worth having; silence is not.
 		now := time.Now()
-		aDry := !r.lastA.IsZero() && now.Sub(r.lastA) > deadSourceAfter
-		bDry := !r.lastB.IsZero() && now.Sub(r.lastB) > deadSourceAfter
-		// Never before either source has spoken at all: at startup both are zero
-		// and neither should be written off.
+		// A source is dry when it has been quiet for too long — and a source that
+		// has NEVER delivered is measured from when capture started, because it
+		// has no last-delivery time of its own.
+		//
+		// The first version required a source to have spoken at least once. That
+		// covers a device that stops, but not the one that actually happens: a
+		// DAX Mic channel that is simply switched off never delivers a single
+		// sample, so it stayed "not yet dry" forever and took the whole recording
+		// down with it. Three empty CW recordings, with the radio audio streaming
+		// perfectly the entire time.
+		dry := func(last time.Time) bool {
+			if last.IsZero() {
+				return now.Sub(r.startedAt) > deadSourceAfter
+			}
+			return now.Sub(last) > deadSourceAfter
+		}
+		aDry, bDry := dry(r.lastA), dry(r.lastB)
 		if bDry && !aDry && len(r.bufA) > 0 {
 			if !r.deadB {
 				r.deadB = true