fix: not update starDifficultyBindable immediately in computeStarRating() to avoid flicker

[hesitling]

• When modifying Mod/Ruleset, the difficulty cache update causes the beatmap difficulty value to change. At this point, the updated value is calculated with Mod and Ruleset applied (the correct new value).
• During PrepareForUse(), it binds to the difficulty cache update, and because runOnceImmediately = true, it immediately triggers an update. At this moment, both the old value and the updated value are the beatmap's base difficulty values (no Mod, no Ruleset).
• The difficulty cache updates again. The old value is the base value, and the new value is the difficulty value calculated with Mod and Ruleset applied.

The flickering occurs because the intermediate PrepareForUse() reverts the difficulty display value back to the beatmap's base difficulty value. Setting runOnceImmediately = false resolves the difficulty value flickering issue.

Resolves #36964.

[peppy]

It feels as though there could be a threading edge case where the value is updated between these two lines, but the threaded retrieval does schedule back to Update thread for the actual updates, so it should be okay.

I'd kinda hope to see test coverage though.

[hesitling]

Maybe another solution.

--- a/osu.Game/Beatmaps/BeatmapDifficultyCache.cs
+++ b/osu.Game/Beatmaps/BeatmapDifficultyCache.cs
@@ -131,10 +131,20 @@ public void Invalidate(IBeatmapInfo oldBeatmap, IBeatmapInfo newBeatmap)
         /// <returns>A bindable that is updated to contain the star difficulty when it becomes available. May be an approximation while in an initial calculating state.</returns>
         public IBindable<StarDifficulty> GetBindableDifficulty(IBeatmapInfo beatmapInfo, CancellationToken cancellationToken = default, int computationDelay = 0)
         {
+            var difficulty = new StarDifficulty(beatmapInfo.StarRating, 0);
+
+            var localBeatmapInfo = beatmapInfo as BeatmapInfo;
+
+            if (localBeatmapInfo != null)
+            {
+                var lookup = new DifficultyCacheLookup(localBeatmapInfo, currentRuleset.Value, currentMods.Value);
+                if (CheckExists(lookup, out var cached) && cached != null)
+                    difficulty = cached.Value;
+            }
             var bindable = new BindableStarDifficulty(beatmapInfo, cancellationToken)
             {
                 // Start with an approximate known value instead of zero.
-                Value = new StarDifficulty(beatmapInfo.StarRating, 0)
+                Value = difficulty
             };

             updateBindable(bindable, currentRuleset.Value, currentMods.Value, cancellationToken, computationDelay);