Cut idle CPU with a busy cluster feed and many windows open

Found by tracing the app with a fake skimmer feed, a fake rigctld and
eight tool windows open (about 3,500 spots on the bandmap).

- Drop expired spots from the one-second spot timer instead of from each
  window's refresh on the UI thread.
- Bandmap keeps the lists On and All return until the spots change, and
  Near searches only the band the frequency is in.
- The Available window replaces its rows at most every two seconds, the
  Score window redraws at most once a second, and the Telnet list takes
  new lines four times a second at most (RateLimited).
- The bandmap leader line pen is made once.

Idle CPU in that setup went from about 33-70% of one core to about 16-25%.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.App/Windows/BandmapView.cs

@@ -199,7 +199,7 @@ public sealed class BandmapView : Control
     {
         IBrush colour = ColourOf(label.Spot);
         context.DrawLine(
-            new Pen(Opaque(ink, 0.55), 1),
+            LeaderPen(ink),
             new Point(ScaleX + 2, label.MarkY),
             new Point(LabelX - 2, label.LabelY + (LabelHeight / 2)));
         context.DrawText(
@@ -285,6 +285,20 @@ public sealed class BandmapView : Control
     private IBrush Foreground() =>
         this.FindResource("FormForeground") as IBrush ?? Brushes.Gray;
 
+    private Pen? leaderPen;
+    private IBrush? leaderInk;
+
+    /// The line from the scale to a callsign. Every label has the same one.
+    private Pen LeaderPen(IBrush ink)
+    {
+        if (leaderPen is null || !ReferenceEquals(leaderInk, ink))
+        {
+            leaderPen = new Pen(Opaque(ink, 0.55), 1);
+            leaderInk = ink;
+        }
+        return leaderPen;
+    }
+
     private readonly Dictionary<(IBrush Ink, double Opacity), IBrush> faded = [];
 
     /// Kept, so the same brush comes back each time and the text drawn with it