debug(icom): say what the CI-V stream was doing when it goes quiet

A real IC-7760 keeps going silent on the CI-V stream two-three minutes into a
session while the control link stays alive. At 10 s of silence the pump now
logs the last real socket error and the retransmit backlog, and notes when the
stream resumes — the 30 s watchdog that follows cannot tell a dead socket from
a rig that stopped talking, and the next occurrence should.

internal/cat/icomnet.go

@@ -359,6 +359,14 @@ func (n *icomNet) civPump() {
 	buf := make([]byte, 8192)
 	lastIdle := time.Now()
 	lastReq := time.Now()
+	// Diagnosis for the recurring 2-3-minute silence a real IC-7760 shows on
+	// this stream while the control link stays alive: when the stream has been
+	// quiet for 10 s, say so ONCE, with the last read error — a socket error
+	// and a rig that stopped talking are different repairs, and the 30 s
+	// watchdog that follows cannot tell them apart from where it sits.
+	lastPkt := time.Now()
+	var lastErr error
+	quietSaid := false
 	for {
 		select {
 		case <-n.done:
@@ -366,7 +374,18 @@ func (n *icomNet) civPump() {
 		default:
 		}
 		_ = n.civ.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
-		if k, err := n.civ.Read(buf); err == nil && k >= 16 {
+		k, err := n.civ.Read(buf)
+		if err != nil {
+			if e, ok := err.(net.Error); !ok || !e.Timeout() {
+				lastErr = err // a REAL socket error, not the read deadline
+			}
+		}
+		if err == nil && k >= 16 {
+			if quietSaid {
+				debugLog.Printf("icom net: CI-V stream is talking again after %s of silence", time.Since(lastPkt).Round(time.Second))
+				quietSaid = false
+			}
+			lastPkt = time.Now()
 			n.markRx()
 			switch typ := icnLE.Uint16(buf[4:]); {
 			case typ == 0x07: // ping
@@ -410,6 +429,10 @@ func (n *icomNet) civPump() {
 				}
 			}
 		}
+		if !quietSaid && time.Since(lastPkt) > 10*time.Second {
+			quietSaid = true
+			debugLog.Printf("icom net: CI-V stream quiet for 10 s (control link still alive; last socket error: %v; missing-seq backlog: %d) — idles and retransmit requests are still being sent", lastErr, len(n.rxMissing))
+		}
 		if time.Since(lastIdle) > 150*time.Millisecond {
 			_, _ = n.civ.Write(icnCtrl(0x00, 0, n.vID, n.vRemote))
 			lastIdle = time.Now()