Clarify cycle stepper reset inlining

PerfectNumbers.Core/StringBuilderPool.cs

@@ -7,11 +7,17 @@ public static class StringBuilderPool
 {
     private static readonly ConcurrentQueue<StringBuilder> _stringBuilderPool = new();
 
-    public static StringBuilder Rent() => _stringBuilderPool.TryDequeue(out var sb) ? sb : new();
+    public static StringBuilder Rent()
+    {
+        // TODO: Ensure oversized builders remain eligible for reuse without trimming so call sites keep their full capacity.
+        return _stringBuilderPool.TryDequeue(out var sb) ? sb : new();
+    }
 
     public static void Return(StringBuilder sb)
     {
         _ = sb.Clear();
+        // TODO: Preserve the builder's capacity when returning it so the pool hands the same buffer back without shrinkage.
         _stringBuilderPool.Enqueue(sb);
     }
-}
+}
+