fix(kenwood): KY takes a FIXED 24 characters, not a string

CW over CAT never worked on a Kenwood. The KY implementation was written against
Elecraft's, which accepts a string of any length, so "KY OH5CX;" went out and a
TS-590SG answered "?;". OpsLog read that as "this radio refuses CW over CAT" and
told the operator to fit a serial keyer — advice that was wrong, and expensive.

The TS-590 manual is explicit: P2 has a fixed length of 24, blanks are filled
with spaces, and those spaces are not keyed. So the fix costs nothing on air; it
is simply the shape the command has. Elecraft stays variable-length, where
padding would key the trailing spaces as word gaps.

The semicolon is also gone from the allowed CW characters. It TERMINATES a CAT
frame, the manual forbids it in P2, and one in a macro would have closed the
command early and left the rest of the message to be read as commands.

Found from a log and a manual page, not from a rig: nobody here owns a Kenwood.
What is proven is the frame shape; that a TS-590SG then keys it still needs the
operator to confirm.

internal/cat/kenwood_cw_test.go

@@ -0,0 +1,42 @@
+package cat
+
+import (
+	"strings"
+	"testing"
+)
+
+// The TS-590 wants a FIXED 24-character P2. Sending the bare text is what made
+// CW over CAT fail on a real TS-590SG: "KY OH5CX;" came back "?;", which OpsLog
+// reported as "this radio rejected CW over CAT" and sent the operator off to buy
+// a serial keyer he did not need.
+func TestKenwoodCWPadsToTheFixedWidth(t *testing.T) {
+	k := &Kenwood{}
+	got := k.padCW("OH5CX")
+	if len(got) != kenwoodCWChunk {
+		t.Errorf("padCW(%q) is %d chars, want exactly %d — the rig rejects anything else",
+			"OH5CX", len(got), kenwoodCWChunk)
+	}
+	if got[:5] != "OH5CX" || strings.TrimRight(got, " ") != "OH5CX" {
+		t.Errorf("padCW = %q — the text must be intact and the rest spaces", got)
+	}
+	// A full piece is already the right width and must not grow.
+	full := strings.Repeat("A", kenwoodCWChunk)
+	if k.padCW(full) != full {
+		t.Error("a full 24-character piece was padded further")
+	}
+
+	// Elecraft takes variable length, and padding there would key the trailing
+	// spaces as word gaps.
+	e := &Kenwood{elecraft: true}
+	if e.padCW("OH5CX") != "OH5CX" {
+		t.Errorf("Elecraft piece was padded: %q", e.padCW("OH5CX"))
+	}
+}
+
+// ";" terminates a CAT frame. The manual forbids it in P2, and one arriving in a
+// macro would close the command early and leave the rest to be read as commands.
+func TestKenwoodCWDropsTheSemicolon(t *testing.T) {
+	if got := filterKenwoodCW("CQ; DE OH5CX"); strings.Contains(got, ";") {
+		t.Errorf("filterKenwoodCW kept a semicolon: %q", got)
+	}
+}