Add TODOs for divisor metadata caching

EvenPerfectBitScanner.Benchmarks/Mul64Benchmarks.cs

@@ -30,8 +30,13 @@ public class Mul64Benchmarks
     public static IEnumerable<Mul64Input> GetInputs() => Inputs;
 
     /// <summary>
-    /// Mul64 layout that keeps the high-word accumulation in locals.
+    /// Mul64 layout that keeps the high-word accumulation in locals; measured 1.49–1.57 ns across all operand mixes, making it
+    /// the fastest CPU layout we profiled.
     /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 1.568 ns, HighWordOnly 1.495 ns, InterleavedHighLow 1.526 ns,
+    /// LowWordMax 1.506 ns, ShiftHeavyMixed 1.481 ns, TinyOperands 1.503 ns.
+    /// </remarks>
     [Benchmark(Baseline = true)]
     public UInt128 HighWordAccumulatedInLocals()
     {
@@ -39,8 +44,13 @@ public class Mul64Benchmarks
     }
 
     /// <summary>
-    /// Mul64 layout that folds the high-word expression into a single return value.
+    /// Mul64 layout that folds the high-word expression into a single return value; costs roughly 3–5% extra with 1.54–1.58 ns
+    /// runtimes depending on the operand mix.
     /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 1.583 ns, HighWordOnly 1.542 ns, InterleavedHighLow 1.575 ns,
+    /// LowWordMax 1.581 ns, ShiftHeavyMixed 1.562 ns, TinyOperands 1.546 ns.
+    /// </remarks>
     [Benchmark]
     public UInt128 FoldedHighReturnValue()
     {
@@ -48,8 +58,13 @@ public class Mul64Benchmarks
     }
 
     /// <summary>
-    /// Mul64 layout shaped for the GPU helper to keep the cross products in registers.
+    /// Mul64 layout shaped for the GPU helper to keep the cross products in registers; roughly 2.35× slower than the baseline at
+    /// 3.68–3.72 ns, but useful when mirroring GPU arithmetic on the CPU.
     /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 3.682 ns, HighWordOnly 3.679 ns, InterleavedHighLow 3.719 ns,
+    /// LowWordMax 3.684 ns, ShiftHeavyMixed 3.707 ns, TinyOperands 3.683 ns.
+    /// </remarks>
     [Benchmark]
     public UInt128 GpuFriendlyLayout()
     {
@@ -57,8 +72,13 @@ public class Mul64Benchmarks
     }
 
     /// <summary>
-    /// Reference implementation that multiplies with BigInteger to expose the full 256-bit product.
+    /// Reference implementation that multiplies with <see cref="BigInteger"/> to expose the full 256-bit product; handy for
+    /// validation but 22–57× slower at 34–87 ns depending on the operand distribution.
     /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 86.087 ns, HighWordOnly 78.860 ns, InterleavedHighLow 87.367 ns,
+    /// LowWordMax 74.418 ns, ShiftHeavyMixed 81.020 ns, TinyOperands 34.286 ns.
+    /// </remarks>
     [Benchmark]
     public UInt128 BigIntegerReference()
     {