fix(lotw): MY_CNTY must not sink a non-US station's upload

TQSL validates MY_CNTY against the ADIF secondary-subdivision list,
which is the US county enumeration — 'XX,County' with a two-letter
state. A Canadian profile produced 'ONTARIO,Kawartha' (the export joined
MY_STATE onto the county wholesale) and TQSL refused the whole record.

Two layers: adifCounty only prefixes a two-letter state, so exports stop
manufacturing the invalid shape; and UploadLoTW scrubs any MY_CNTY that
is not the US shape before signing — MY_STATE and MY_GRIDSQUARE already
locate the station for LoTW, and the US form survives for the county
hunters. Table-tested.

internal/extsvc/lotw_scrub_test.go

@@ -0,0 +1,23 @@
+package extsvc
+
+import "testing"
+
+// TQSL refuses whole records over a MY_CNTY it cannot validate, and its
+// validation is the US "XX,County" enumeration — so anything else must be
+// stripped before signing, and the US shape must survive untouched.
+func TestScrubMyCnty(t *testing.T) {
+	cases := []struct{ in, want string }{
+		{"<CALL:5>F4BPO<MY_CNTY:16>ONTARIO,Kawartha<MY_STATE:7>ONTARIO<EOR>",
+			"<CALL:5>F4BPO<MY_STATE:7>ONTARIO<EOR>"},
+		{"<MY_CNTY:8>Kawartha<EOR>", "<EOR>"},
+		{"<MY_CNTY:9>NY,Monroe<EOR>", "<MY_CNTY:9>NY,Monroe<EOR>"},
+		{"<CALL:4>K1AB<EOR>", "<CALL:4>K1AB<EOR>"},
+		{"<MY_CNTY:8>Kawartha<EOR>\n<MY_CNTY:9>NY,Monroe<EOR>",
+			"<EOR>\n<MY_CNTY:9>NY,Monroe<EOR>"},
+	}
+	for _, c := range cases {
+		if got := scrubMyCnty(c.in); got != c.want {
+			t.Errorf("scrubMyCnty(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}