fix(dxcc): /MM and /AM belong to no entity

RI1FJL/MM resolved to Franz Josef Land while the expedition was still sailing
there, telling the operator they had worked an entity they had not.

The old behaviour was deliberate — the comment read "strict DXCC says no
entity, but the log should still show the operator's country" — and that is
wrong for exactly this reason. A home country on a maritime mobile is not extra
information; it is a false claim about where the contact happened, and it is
the kind of false claim that gets a QSO submitted for an award it cannot win.

Trailing only. A LEADING "MM" is the Scotland prefix and "AM" is Spain, so
MM0ABC and AM5X keep their entities — getting that wrong would be a far larger
error than the one being fixed. /P, /M and /B are unaffected: a portable
station is still ashore.

In the cluster the column says "Maritime mobile — no DXCC" rather than going
blank, because blank is what an UNRESOLVED spot looks like and the two mean
opposite things — one is "we don't know yet", this is "there is nothing to
know". Display only: the QSO's own Country stays empty, since that field holds
a DXCC entity name and there is none.

internal/dxcc/maritime_test.go

@@ -0,0 +1,39 @@
+package dxcc
+
+import "testing"
+
+// /MM and /AM belong to NO DXCC entity: the station is at sea or in the air.
+//
+// This is not a technicality. RI1FJL/MM resolved to Franz Josef Land while the
+// expedition was still sailing there, which told the operator they had worked
+// an entity they had not.
+func TestMobileNoEntity(t *testing.T) {
+	for _, tc := range []struct {
+		call string
+		want bool
+	}{
+		{"RI1FJL/MM", true},
+		{"YB1SCY/AM", true},
+		{"ri1fjl/mm", true},
+		{" RI1FJL/MM ", true},
+		// A LEADING MM is Scotland and AM is Spain. Treating those as
+		// entity-less would be a far larger error than the one this fixes.
+		{"MM0ABC", false},
+		{"AM5X", false},
+		{"MM/F4BPO", false},
+		{"AM/DL1ABC", false},
+		// Other suffixes keep their entity — a portable station is still ashore.
+		{"F4BPO/P", false},
+		{"F4BPO/M", false},
+		{"4U1UN/B", false},
+		{"F4BPO", false},
+		// Only the LAST element counts: RI1FJL/MM/P is still maritime, and a
+		// call whose /MM is not final is not.
+		{"RI1FJL/MM/P", false},
+		{"", false},
+	} {
+		if got := IsMobileNoEntity(tc.call); got != tc.want {
+			t.Errorf("IsMobileNoEntity(%q) = %v, want %v", tc.call, got, tc.want)
+		}
+	}
+}