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.Spotting/Bandmap.cs

@@ -20,6 +20,13 @@ public sealed class Bandmap
     private readonly Lock guard = new();
     private readonly Func<DateTime> clock;
 
+    /// The lists `On` and `All` hand out, kept until the next change. The
+    /// windows ask on every refresh, a refresh can come many times a second
+    /// while the spots change once a second, and each answer is a scan and a
+    /// sort of every spot.
+    private readonly Dictionary<Band, IReadOnlyList<Spot>> onBand = [];
+    private IReadOnlyList<Spot>? everything;
+
     public Bandmap(TimeSpan? lifetime = null, Func<DateTime>? clock = null)
     {
         Lifetime = lifetime ?? TimeSpan.FromHours(1);
@@ -43,6 +50,7 @@ public sealed class Bandmap
         {
             spots[key] = spot;
             arrived[key] = clock();
+            Forget();
         }
         Changed?.Invoke(this, EventArgs.Empty);
     }
@@ -61,6 +69,7 @@ public sealed class Bandmap
                 arrived[key] = clock();
                 added++;
             }
+            Forget();
         }
         if (added > 0)
         {
@@ -76,6 +85,7 @@ public sealed class Bandmap
         {
             arrived.Remove(key);
             removed = spots.Remove(key);
+            Forget();
         }
         if (removed)
         {
@@ -99,6 +109,10 @@ public sealed class Bandmap
                 arrived.Remove(key);
             }
             dropped = stale.Count;
+            if (dropped > 0)
+            {
+                Forget();
+            }
         }
         if (dropped > 0)
         {
@@ -119,14 +133,26 @@ public sealed class Bandmap
         }
     }
 
+    /// Called under the lock by everything that changes the spots.
+    private void Forget()
+    {
+        onBand.Clear();
+        everything = null;
+    }
+
     public IReadOnlyList<Spot> On(Band band)
     {
         lock (guard)
         {
-            return spots.Values
-                .Where(s => s.Band == band)
-                .OrderBy(s => s.Frequency.Hertz)
-                .ToList();
+            if (!onBand.TryGetValue(band, out IReadOnlyList<Spot>? found))
+            {
+                found = spots.Values
+                    .Where(s => s.Band == band)
+                    .OrderBy(s => s.Frequency.Hertz)
+                    .ToList();
+                onBand[band] = found;
+            }
+            return found;
         }
     }
 
@@ -134,7 +160,7 @@ public sealed class Bandmap
     {
         lock (guard)
         {
-            return spots.Values.OrderBy(s => s.Frequency.Hertz).ToList();
+            return everything ??= spots.Values.OrderBy(s => s.Frequency.Hertz).ToList();
         }
     }
 
@@ -146,7 +172,12 @@ public sealed class Bandmap
         {
             Spot? best = null;
             long bestDistance = long.MaxValue;
-            foreach (Spot spot in spots.Values)
+            // only the band the frequency is in is searched, which is a few
+            // hundred spots rather than all of them
+            IEnumerable<Spot> candidates = Bands.ForFrequency(frequency) is { } band
+                ? On(band)
+                : spots.Values;
+            foreach (Spot spot in candidates)
             {
                 long distance = Math.Abs(spot.Frequency.Hertz - frequency.Hertz);
                 if (distance <= window.Hertz && distance < bestDistance)