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.go

@@ -29,7 +29,30 @@ const kenwoodCWChunk = 24
 
 // kenwoodCWAllowed is what the keyer can send; anything else is dropped, since an
 // unsupported byte can abort the buffer and lose the rest of the message.
-const kenwoodCWAllowed = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 /?.,-=+:;()"
+// The semicolon is deliberately absent: it TERMINATES a CAT frame. The TS-590
+// manual says so outright for P2, and one in a macro would close the command
+// early and leave the rest of the text to be read as commands of its own.
+const kenwoodCWAllowed = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 /?.,-=+:()"
+
+// padCW pads a piece to the fixed 24-character P2 a Kenwood demands.
+//
+// This is where CW over CAT failed on a TS-590SG. The KY command was written
+// against Elecraft's, which takes a variable-length string, so "KY OH5CX;" went
+// out and the radio answered "?;" — read as "this rig refuses CW over CAT", and
+// the operator was told to go and buy a serial keyer.
+//
+// The TS-590 manual is explicit: P2 has a FIXED length of 24, and characters
+// left blank are filled with spaces which are NOT keyed. So the padding costs
+// nothing on air — it is simply the shape the command has to have.
+//
+// Elecraft is left variable-length: it accepts short strings, and padding there
+// would key the trailing spaces as word gaps.
+func (k *Kenwood) padCW(chunk string) string {
+	if k.elecraft || len(chunk) >= kenwoodCWChunk {
+		return chunk
+	}
+	return chunk + strings.Repeat(" ", kenwoodCWChunk-len(chunk))
+}
 
 // SendCW queues a message on the rig's keyer, fed in 24-character pieces, waiting
 // for buffer room between pieces so a long macro doesn't lose its tail.
@@ -51,7 +74,7 @@ func (k *Kenwood) SendCW(text string) error {
 		chunk := msg[:n]
 		msg = msg[n:]
 		k.waitCWBuffer(3 * time.Second)
-		if err := k.write("KY " + chunk + ";"); err != nil {
+		if err := k.write("KY " + k.padCW(chunk) + ";"); err != nil {
 			return err
 		}
 		if err := k.afterKY(); err != nil {