Show hit error on results screen

Leading up to implementation of "local offset", this feels like a good
thing to have visible first and foremost.

osu.Game.Rulesets.Mania/ManiaRuleset.cs

@@ -394,6 +394,7 @@ namespace osu.Game.Rulesets.Mania
                 {
                     new StatisticItem(string.Empty, () => new SimpleStatisticTable(3, new SimpleStatisticItem[]
                     {
+                        new AverageHitError(score.HitEvents),
                         new UnstableRate(score.HitEvents)
                     }), true)
                 }

osu.Game.Rulesets.Osu/OsuRuleset.cs

@@ -314,6 +314,7 @@ namespace osu.Game.Rulesets.Osu
                     {
                         new StatisticItem(string.Empty, () => new SimpleStatisticTable(3, new SimpleStatisticItem[]
                         {
+                            new AverageHitError(timedHitEvents),
                             new UnstableRate(timedHitEvents)
                         }), true)
                     }

osu.Game.Rulesets.Taiko/TaikoRuleset.cs

@@ -237,6 +237,7 @@ namespace osu.Game.Rulesets.Taiko
                     {
                         new StatisticItem(string.Empty, () => new SimpleStatisticTable(3, new SimpleStatisticItem[]
                         {
+                            new AverageHitError(timedHitEvents),
                             new UnstableRate(timedHitEvents)
                         }), true)
                     }

osu.Game/Rulesets/Scoring/HitEventExtensions.cs

@@ -22,6 +22,9 @@ namespace osu.Game.Rulesets.Scoring
             return 10 * standardDeviation(timeOffsets);
         }
 
+        public static double? CalculateAverageHitError(this IEnumerable<HitEvent> hitEvents) =>
+            hitEvents.Where(affectsUnstableRate).Select(ev => ev.TimeOffset).Average();
+
         private static bool affectsUnstableRate(HitEvent e) => !(e.HitObject.HitWindows is HitWindows.EmptyHitWindows) && e.Result.IsHit();
 
         private static double? standardDeviation(double[] timeOffsets)

osu.Game/Screens/Ranking/Statistics/AverageHitError.cs

@@ -0,0 +1,27 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using osu.Game.Rulesets.Scoring;
+
+namespace osu.Game.Screens.Ranking.Statistics
+{
+    /// <summary>
+    /// Displays the unstable rate statistic for a given play.
+    /// </summary>
+    public class AverageHitError : SimpleStatisticItem<double?>
+    {
+        /// <summary>
+        /// Creates and computes an <see cref="AverageHitError"/> statistic.
+        /// </summary>
+        /// <param name="hitEvents">Sequence of <see cref="HitEvent"/>s to calculate the unstable rate based on.</param>
+        public AverageHitError(IEnumerable<HitEvent> hitEvents)
+            : base("Average Hit Error")
+        {
+            Value = hitEvents.CalculateAverageHitError();
+        }
+
+        protected override string DisplayValue(double? value) => value == null ? "(not available)" : $"{Math.Abs(value.Value):N2} ms {(value.Value < 0 ? "early" : "late")}";
+    }
+}