fix: a CW macro knocked the Yaesu CAT link over

Clicking a macro dropped the CAT for a few seconds, keyed nothing, then came
back.

ask() returned the first ';'-terminated frame it saw, whatever command it
belonged to. KY produces NO reply, so the next query — FA; from the poll loop —
collected a leftover frame, failed to parse it as a frequency, and ReadState
reported an error. The Manager reads that as "lost the rig": disconnect, wait,
reconnect. Hence the drop and the automatic recovery a few seconds later.

Replies are now matched to the command that asked for them: anything else is
discarded and logged, so a stray frame costs one log line instead of the link.

This also explains the silence — the send never got a clean run at the port
while the backend was being torn down under it.

internal/cat/yaesu_test.go

@@ -103,3 +103,30 @@ func TestYaesuModeDigit(t *testing.T) {
 		}
 	}
 }
+
+// A reply belongs to the command that asked for it.
+//
+// Without this, a CW macro knocked the CAT link over: KY produces no reply, so
+// the poll loop's next FA; collected a leftover frame, failed to parse it as a
+// frequency, and the Manager treated that as "lost the rig" and reconnected —
+// the CAT dropping for a few seconds on every macro click.
+func TestYaesuCmdPrefix(t *testing.T) {
+	cases := []struct{ cmd, want string }{
+		{"FA;", "FA"},
+		{"FB;", "FB"},
+		{"MD0;", "MD"},
+		{"KY;", "KY"},
+		{"KY CQ TEST;", "KY"},
+		{"SM0;", "SM"},
+		{"RM4;", "RM"},
+		{"AG0;", "AG"},
+		{"TX;", "TX"},
+		{"", ""},
+		{";", ""},
+	}
+	for _, c := range cases {
+		if got := cmdPrefix(c.cmd); got != c.want {
+			t.Errorf("cmdPrefix(%q) = %q, want %q", c.cmd, got, c.want)
+		}
+	}
+}