Document GPU kernel audit progress

PerfectNumbers.Core/Gpu/Kernels/NttTransformKernels.cs

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+using ILGPU;
+
+namespace PerfectNumbers.Core.Gpu;
+
+internal static class NttTransformKernels
+{
+    public static void ForwardKernel(Index1D index, ArrayView<GpuUInt128> input, ArrayView<GpuUInt128> output, int length, GpuUInt128 modulus, GpuUInt128 root)
+    {
+        // TODO(NTT-OPT): Replace this reference O(n^2) kernel with a stage-wise
+        // Cooley–Tukey butterfly kernel (O(n log n)). This version uses a per-
+        // element ModPow in the hot loop which is far too slow and causes long
+        // single-kernel runtimes. After twiddle precomputation, each butterfly
+        // should perform: (u, v) -> (u+v*w, u-v*w) with a single MulMod.
+        var sum = new GpuUInt128(0UL, 0UL);
+        for (int i = 0; i < length; i++)
+        {
+            var term = input[i];
+            var power = new GpuUInt128(root);
+            power.ModPow((ulong)index.X * (ulong)i, modulus);
+            term.MulMod(power, modulus);
+            sum.AddMod(term, modulus);
+        }
+
+        output[index] = sum;
+    }
+
+    public static void InverseKernel(Index1D index, ArrayView<GpuUInt128> input, ArrayView<GpuUInt128> output, int length, GpuUInt128 modulus, GpuUInt128 rootInv, GpuUInt128 nInv)
+    {
+        // TODO(NTT-OPT): Replace this O(n^2) inverse with stage-wise butterflies
+        // using precomputed inverse twiddles. Apply final scaling by nInv after
+        // all stages, preferably in a small dedicated kernel.
+        var sum = new GpuUInt128(0UL, 0UL);
+        for (int i = 0; i < length; i++)
+        {
+            var term = input[i];
+            var power = new GpuUInt128(rootInv);
+            power.ModPow((ulong)index.X * (ulong)i, modulus);
+            term.MulMod(power, modulus);
+            sum.AddMod(term, modulus);
+        }
+
+        sum.MulMod(nInv, modulus);
+        output[index] = sum;
+    }
+
+    public static void BitReverseKernel(Index1D index, ArrayView<GpuUInt128> data, int bits)
+    {
+        int i = index.X;
+        int j = ReverseBits(i, bits);
+        if (i < j)
+        {
+            var tmp = data[i];
+            data[i] = data[j];
+            data[j] = tmp;
+        }
+    }
+
+    internal static int ReverseBits(int value, int bits)
+    {
+        int result = 0;
+        for (int i = 0; i < bits; i++)
+        {
+            result = (result << 1) | (value & 1);
+            value >>= 1;
+        }
+
+        return result;
+    }
+}