fix(udp): ignore N0CALL, and stop returning a grid as a callsign

N0CALL is what WSJT-X transmits under when its owner never set a callsign. It
has a letter, a digit and an ordinary shape, so nothing rejected it: it was
spotted, coloured, counted as a new WPX prefix, and now that CQ grids are read
it would have put a grid into the worked index under a callsign nobody holds.

The obvious fix - adding it to looksLikeCall's reject list - was wrong, and the
test caught it before it shipped. That function answers "could this token be a
callsign at all", and the CQ grammar uses it to decide whether the word after CQ
is a modifier (DX, NA, a zone) or the call itself. Teaching it that N0CALL is
not a callsign made "CQ N0CALL JN36" skip a slot and return JN36. Shape and
policy are different questions and now live in different functions.

Chasing that turned up the real defect behind it: ANY unrecognised word after CQ
made the parser skip a slot, and a four-character grid passes every shape test a
callsign does. "CQ FOO JN36" returned JN36 as the sender - logged, spotted and
coloured as a station. A grid in the callsign slot is now refused outright.

internal/integrations/udp/n1mm_test.go

@@ -37,19 +37,19 @@ func TestParseN1MMContactInfo(t *testing.T) {
 		t.Fatal("expected a loggable contact, got ok=false")
 	}
 	want := map[string]string{
-		"<call:5>VE9AA":     "callsign",
+		"<call:5>VE9AA":        "callsign",
 		"<qso_date:8>20240315": "date",
-		"<time_on:6>142530": "time",
-		"<band:3>20m":       "band",
-		"<mode:2>CW":        "mode",
-		"<freq:9>14.025000": "freq",
-		"<rst_sent:3>599":   "rst sent",
-		"<rst_rcvd:3>599":   "rst rcvd",
-		"<gridsquare:4>FN65": "grid",
-		"<name:4>Mike":      "name",
-		"<stx:1>1":          "stx serial",
-		"<srx:2>42":         "srx serial",
-		"<eor>":             "terminator",
+		"<time_on:6>142530":    "time",
+		"<band:3>20m":          "band",
+		"<mode:2>CW":           "mode",
+		"<freq:9>14.025000":    "freq",
+		"<rst_sent:3>599":      "rst sent",
+		"<rst_rcvd:3>599":      "rst rcvd",
+		"<gridsquare:4>FN65":   "grid",
+		"<name:4>Mike":         "name",
+		"<stx:1>1":             "stx serial",
+		"<srx:2>42":            "srx serial",
+		"<eor>":                "terminator",
 	}
 	for sub, label := range want {
 		if !strings.Contains(adif, sub) {