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.

internal/catemu/catemu_test.go

@@ -0,0 +1,45 @@
+package catemu
+
+import "testing"
+
+// The reply LENGTHS are the contract: an amplifier parses these frames by
+// fixed offsets, so a frame one character short desynchronises its parser for
+// every following poll. Pin them.
+func TestReplyShapes(t *testing.T) {
+	s := New(Config{ComPort: "COM99", Baud: 9600}, nil)
+	s.SetFrequency(14025000)
+	s.SetMode("CW")
+
+	if got := s.ifFrame(14025000); len(got) != 38 {
+		t.Errorf("IF frame is %d chars, want 38: %q", len(got), got)
+	}
+	// TS-2000 IF: "IF" then the 11-digit frequency in Hz.
+	if got := s.ifFrame(14025000); got[:13] != "IF00014025000" {
+		t.Errorf("IF frame frequency field = %q", got[:13])
+	}
+	if got := s.ifFrame(14025000); got[len(got)-1] != ';' {
+		t.Errorf("IF frame not terminated by ';': %q", got)
+	}
+	// Mode digit sits at P9, right after freq(11)+step(4)+rit(6)+3+2+1 — index 29 in the frame.
+	if got := s.ifFrame(14025000)[29]; got != '3' {
+		t.Errorf("CW mode digit = %q, want '3'", got)
+	}
+	s.SetMode("FT8") // data rides on SSB as far as an amp cares
+	if got := s.ifFrame(14074000)[29]; got != '2' {
+		t.Errorf("FT8 mode digit = %q, want '2'", got)
+	}
+}
+
+func TestModeDigit(t *testing.T) {
+	cases := map[string]byte{
+		"CW": '3', "CW-R": '3', "LSB": '1', "USB": '2', "SSB": '2',
+		"FM": '4', "AM": '5', "RTTY": '6', "": '2', "FT8": '2',
+	}
+	for mode, want := range cases {
+		s := New(Config{}, nil)
+		s.SetMode(mode)
+		if got := s.modeDigit(); got != want {
+			t.Errorf("mode %q → %q, want %q", mode, got, want)
+		}
+	}
+}