feat: CW through the Yaesu keyer — fifth CW engine

The radio has a keyer and a command to feed it (KY), so an FTDX10 needs no
WinKeyer and no second cable, exactly as the Icom CI-V and Flex CWX engines
already do. The rig keys with its own timing, which is why the spacing is right
where a PC keying a line through USB latency drifts.

Text is filtered to what the keyer can actually send: an unsupported byte does
not produce an error on a Yaesu, it can abort the whole buffer, so the rest of a
macro would vanish silently. It is then fed in 24-character pieces, waiting for
room between them — the rig DROPS what does not fit, again with no error, so a
contest CQ would lose its tail.

STOP is the honest gap. Yaesu documents no buffer-clear, so it drops the
transmitter (TX0) instead: nothing queued reaches the air, which is what Escape
means to an operator. It deliberately does NOT send "KY0;" — a plausible-looking
clear that the rig would read as the CHARACTER zero and transmit.

A test caught a real one on the way: tabs and newlines were dropped as
"unsupported", gluing the words either side together, so a macro written on two
lines went out as CQCQ. Whitespace now becomes a word gap before filtering.

Settings warn when the Yaesu keyer is selected without the Yaesu CAT backend —
otherwise it simply never keys, with nothing on screen saying why.

Send path follows the CAT reference and how Hamlib drives these rigs; NOT yet
verified on the air.

internal/cat/yaesu_cw_test.go

@@ -0,0 +1,63 @@
+package cat
+
+import (
+	"strings"
+	"testing"
+)
+
+// What reaches the rig's keyer.
+//
+// An unsupported byte does not produce an error on a Yaesu — it can abort the
+// whole buffer, so the rest of the message is lost silently. Filtering is
+// therefore part of correctness, not tidiness.
+func TestFilterYaesuCW(t *testing.T) {
+	cases := []struct{ in, want string }{
+		{"cq cq de f4bpo", "CQ CQ DE F4BPO"},
+		{"F4BPO/P", "F4BPO/P"},
+		{"599 tu 73", "599 TU 73"},
+		// Accents and symbols the keyer has no code for.
+		{"héllo", "HLLO"},
+		{"a*b#c", "ABC"},
+		// Runs of whitespace collapse: the rig sends each space as a word gap, so
+		// three in a row is three gaps and the message crawls.
+		{"cq    cq", "CQ CQ"},
+		{"  padded  ", "PADDED"},
+		{"\tcq\ncq\t", "CQ CQ"},
+		{"", ""},
+		{"   ", ""},
+	}
+	for _, c := range cases {
+		if got := filterYaesuCW(c.in); got != c.want {
+			t.Errorf("filterYaesuCW(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}
+
+// Long macros have to be split: one KY command carries 24 characters and the rig
+// DROPS what does not fit rather than reporting an error, so a contest CQ would
+// lose its tail with no sign anywhere.
+func TestYaesuCWChunking(t *testing.T) {
+	msg := filterYaesuCW("cq cq cq de f4bpo f4bpo f4bpo pse k")
+	if len(msg) <= yaesuCWChunk {
+		t.Fatalf("test message is %d chars — too short to exercise chunking", len(msg))
+	}
+	var chunks []string
+	for rest := msg; len(rest) > 0; {
+		n := yaesuCWChunk
+		if len(rest) < n {
+			n = len(rest)
+		}
+		chunks = append(chunks, rest[:n])
+		rest = rest[n:]
+	}
+	for i, c := range chunks {
+		if len(c) > yaesuCWChunk {
+			t.Errorf("chunk %d is %d chars, over the %d limit", i, len(c), yaesuCWChunk)
+		}
+	}
+	// Nothing added, nothing lost: the message the operator typed is what the
+	// keyer receives.
+	if joined := strings.Join(chunks, ""); joined != msg {
+		t.Errorf("chunks rejoin to %q, want %q", joined, msg)
+	}
+}