Eliminate GC allocations from CPU divisor scan hot path

PerfectNumbers.Core/Cpu/FactorCacheLease.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+
+namespace PerfectNumbers.Core.Cpu;
+
+internal struct FactorCacheLease
+{
+    private Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? _cache;
+
+    public void EnsureInitialized(ref Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache)
+    {
+        Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? current = cache;
+        if (current is null)
+        {
+            current = ThreadStaticPools.RentMersenneFactorCacheDictionary();
+            cache = current;
+        }
+
+        _cache = current;
+    }
+
+    public void Dispose()
+    {
+        Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache = _cache;
+        if (cache is null)
+        {
+            return;
+        }
+
+        ThreadStaticPools.ReturnMersenneFactorCacheDictionary(cache);
+        _cache = null;
+    }
+}