feat: amplifier band-follow — answer the amp's frequency polls

On its CAT/AUX connector an ACOM is the MASTER: it polls a transceiver and
changes band from the reply, so nothing can be pushed to it. internal/catemu
answers those polls on a second serial port, in ACOM command set 5 (Kenwood /
Elecraft): FA;, FB;, IF; and ID;, and nothing else — a wrong-length reply is
worse than none, it desynchronises the amp's parser for the following poll.

Also offered for SPE. Some amps do not poll at all but read the radio↔PC CAT
line in parallel; those never hear a responder, so an optional unprompted send
(500/1000 ms) covers them.

The TX frequency is what is sent: in split the amp must be tuned where we
transmit. Frame lengths are pinned by tests — the failure they guard against is
silent and only shows up as an amp that mistunes.

Untested on hardware.

app.go

@@ -30,6 +30,7 @@ import (
 	"hamlog/internal/backup"
 	"hamlog/internal/cabrillo"
 	"hamlog/internal/cat"
+	"hamlog/internal/catemu"
 	"hamlog/internal/clublog"
 	"hamlog/internal/cluster"
 	"hamlog/internal/contest"
@@ -917,6 +918,10 @@ func (a *App) startup(ctx context.Context) {
 			wruntime.EventsEmit(a.ctx, "cat:state", s)
 		}
 		a.emitRadioUDP(s)
+		// Feed the frequency to any amplifier we are pretending to be a radio
+		// for. Just two atomic stores per amp — the reply itself is built when
+		// the amp polls, so a fast-tuning VFO costs nothing here.
+		a.feedAmpBandFollow(s)
 		// Drive station relays by the current frequency/band (PstRotator-style
 		// automatic control). Cheap cached-flag check keeps this a no-op when the
 		// feature is off; when on, run off this callback so a slow relay board never
@@ -13343,13 +13348,26 @@ type AmpConfig struct {
 	Port      int    `json:"port"`
 	ComPort   string `json:"com_port"`
 	Baud      int    `json:"baud"`
+
+	// Band-follow: an ACOM is the MASTER on its CAT/AUX connector — it polls a
+	// transceiver and changes band from the reply, so following OpsLog means
+	// answering those polls on a SECOND serial port (independent of the one used
+	// above for metering; both run at once). See internal/catemu.
+	FreqOut     bool   `json:"freq_out"`
+	FreqComPort string `json:"freq_com_port"`
+	FreqBaud    int    `json:"freq_baud"`
+	// FreqBroadcastMs > 0 also SENDS the frequency unprompted at that interval,
+	// for an amplifier that listens to a transceiver's CAT stream instead of
+	// polling it. 0 = answer polls only.
+	FreqBroadcastMs int `json:"freq_broadcast_ms"`
 }
 
 type ampInst struct {
-	cfg  AmpConfig
-	pgxl *powergenius.Client
-	spe  *spe.Client
-	acom *acom.Client
+	cfg    AmpConfig
+	pgxl   *powergenius.Client
+	spe    *spe.Client
+	acom   *acom.Client
+	catemu *catemu.Server // Kenwood-format responder for band-follow (ACOM)
 }
 
 func (i *ampInst) stopAll() {
@@ -13362,6 +13380,9 @@ func (i *ampInst) stopAll() {
 	if i.acom != nil {
 		i.acom.Stop()
 	}
+	if i.catemu != nil {
+		i.catemu.Stop()
+	}
 }
 
 // ampTypeLabel is the default display name for an amp type.
@@ -13491,12 +13512,61 @@ func (a *App) startAmps() {
 				a.spe = inst.spe
 			}
 		}
+		// Band-follow on a SECOND serial port, for any amp that takes its band from
+		// a transceiver CAT link (ACOM and SPE both do). Independent of the control
+		// transport above, so metering and band-follow run at the same time.
+		if c.FreqOut && strings.TrimSpace(c.FreqComPort) != "" {
+			inst.catemu = catemu.New(catemu.Config{
+				ComPort: c.FreqComPort, Baud: c.FreqBaud,
+				BroadcastMs: c.FreqBroadcastMs,
+			}, applog.Printf)
+			if st := a.cat.State(); st.Connected {
+				inst.catemu.SetFrequency(st.FreqHz)
+				inst.catemu.SetMode(st.Mode)
+			}
+			inst.catemu.Start()
+			applog.Printf("amp %s: band-follow on %s at %d baud (Kenwood format, broadcast=%dms)",
+				c.Name, c.FreqComPort, c.FreqBaud, c.FreqBroadcastMs)
+		}
 		a.ampsMu.Lock()
 		a.ampInsts[c.ID] = inst
 		a.ampsMu.Unlock()
 	}
 }
 
+// feedAmpBandFollow pushes the rig's frequency/mode to every amplifier we are
+// emulating a transceiver for. Called on each CAT state change.
+//
+// The TX frequency is what the amp must follow: in split it has to be tuned
+// for where we transmit, not where we listen.
+func (a *App) feedAmpBandFollow(s cat.RigState) {
+	if !s.Connected || s.FreqHz <= 0 {
+		return
+	}
+	a.ampsMu.Lock()
+	defer a.ampsMu.Unlock()
+	for _, inst := range a.ampInsts {
+		if inst.catemu != nil {
+			inst.catemu.SetFrequency(s.FreqHz)
+			inst.catemu.SetMode(s.Mode)
+		}
+	}
+}
+
+// GetAmpBandFollowStatus returns the band-follow link state per amplifier id,
+// so the settings panel can show whether the amp is actually polling us.
+func (a *App) GetAmpBandFollowStatus() map[string]catemu.Status {
+	out := map[string]catemu.Status{}
+	a.ampsMu.Lock()
+	defer a.ampsMu.Unlock()
+	for id, inst := range a.ampInsts {
+		if inst.catemu != nil {
+			out[id] = inst.catemu.GetStatus()
+		}
+	}
+	return out
+}
+
 // AmpStatus is one amp's live state for the UI poll — exactly one of the
 // per-family payloads is set, per the amp's type.
 type AmpStatus struct {