Draw the grey line map

Window > Grey Line opens the world with the night on it. The dark area is
where the sun is down, and the band along its edge is the twilight from the
horizon to six degrees below it, which is the grey line an operator watches
for on the low bands.

Grayline works out where the sun stands: the subsolar point, the elevation at
a place, and the latitude the terminator crosses a meridian at, which is what
the map fills the night between. It is the same question SunTimes answers from
the other end, and the two are checked against each other in the tests.

The coastlines are Natural Earth's 110m coastline, public domain, cut to a
tenth of a degree.

Our own station is drawn from the grid square, the station being called and
the spots on the band from the country file. The map follows the clock.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.App/Windows/EntryWindow.Menu.cs

@@ -428,6 +428,10 @@ public sealed partial class EntryWindow
 
     private void OnShowScore(object? sender, RoutedEventArgs e) => Show(() => new ScoreWindow(session));
 
+    /// N1MM's grey line window: where the daylight is now, and where it will be.
+    private void OnShowGrayline(object? sender, RoutedEventArgs e) =>
+        Show(() => new GraylineWindow(session));
+
     private void OnShowTelnet(object? sender, RoutedEventArgs e) => Show(() => new TelnetWindow(session, Tune));
 
     /// N1MM's digital interface window, which is where RTTY is worked from.