fix(bandopen): the moon is not a band opening

A 2 m opening was announced at 9650 km towards Japan. The callsigns gave it
away — 7M4RRM, JA7RPC, JF1AWC — all working EME, which is routine on 2 m and
reported to PSK Reporter like anything else. Real contacts, real grids, and no
evidence whatsoever about the band: an operator turning a beam on that bearing
would hear nothing.

Removing the flat 2400 km ceiling was right; it threw away genuine multi-hop Es
on 6 m. "No limit anywhere" was the overcorrection. The limit is per band now,
and it is physics rather than a threshold: 2 m reaches a few thousand kilometres
by tropospheric duct or a chain of Es clouds and no further, so 3500 km, and
4 m 4000. Six and ten metres keep no ceiling — multi-hop Es and F2 genuinely do
go round the world, which is the case that started all this.

Pinned from both sides: the 9650 km Japanese burst produces nothing, and a
2000 km 2 m burst in one sector still counts.

internal/bandopen/eme_test.go

@@ -0,0 +1,51 @@
+package bandopen
+
+import (
+	"testing"
+	"time"
+)
+
+// A 2 m "opening" was announced at 9650 km towards Japan on stations that were
+// plainly working EME. The moon is not an opening: an operator pointing an
+// antenna at that bearing finds nothing.
+func TestEMEIsNotAnOpeningOnTwoMetres(t *testing.T) {
+	d := New(DefaultConfig())
+	base := time.Date(2026, 8, 12, 6, 0, 0, 0, time.UTC)
+	for i, call := range []string{"7M4RRM", "JA7RPC", "JF1AWC", "JK1TPA", "JH1JCQ"} {
+		if op := d.Add(Spot{
+			Call: call, Band: "2m", DistKm: 9650, Bearing: 40 + i*3,
+			At: base.Add(time.Duration(i) * time.Minute),
+		}, 47.0); op != nil {
+			t.Fatalf("a 9650 km 2 m path was announced as an opening: %+v", op)
+		}
+	}
+}
+
+// But a real 2 m opening — an Es chain at a plausible distance — must survive.
+func TestLongButPlausibleTwoMetresStillCounts(t *testing.T) {
+	d := New(DefaultConfig())
+	base := time.Date(2026, 6, 20, 18, 0, 0, 0, time.UTC)
+	var got *Opening
+	for i, call := range []string{"EA1AA", "CT1BB", "EA7CC", "CT7DD"} {
+		if op := d.Add(Spot{
+			Call: call, Band: "2m", DistKm: 2000 + i*30, Bearing: 200 + i*4,
+			At: base.Add(time.Duration(i) * time.Minute),
+		}, 47.0); op != nil {
+			got = op
+		}
+	}
+	if got == nil {
+		t.Fatal("a 2000 km 2 m burst in one sector was not reported")
+	}
+}
+
+// Six metres keeps no ceiling: multi-hop Es genuinely goes that far, which is
+// why the flat limit was removed in the first place.
+func TestSixMetresHasNoCeiling(t *testing.T) {
+	if MaxKmFor("6m") != 0 || MaxKmFor("10m") != 0 {
+		t.Error("6 m and 10 m must have no distance ceiling")
+	}
+	if MaxKmFor("2m") == 0 {
+		t.Error("2 m must have one")
+	}
+}