Document GPU kernel audit progress

PerfectNumbers.Core/Gpu/Kernels/NttButterflyKernels.cs

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+using ILGPU;
+
+namespace PerfectNumbers.Core.Gpu;
+
+internal static class NttButterflyKernels
+{
+    public static void StageKernel(Index1D index, ArrayView<GpuUInt128> data, int len, int half, int stageOffset, ArrayView<GpuUInt128> twiddles, GpuUInt128 modulus)
+    {
+        // TODO(NTT-OPT): Consider shared-memory tiling to improve locality:
+        // load a block of size `len` into shared memory, do butterflies, write back.
+        // Requires explicit grouped kernels and chosen group size.
+        int t = index.X;
+        // TODO: Replace this `%` with a bitmask when `half` is a power of two so the stage index math matches the faster
+        // residue strategy from the GPU residue benchmarks.
+        int j = t % half;
+        int block = t / half;
+        int k = block * len;
+        int i1 = k + j;
+        int i2 = i1 + half;
+        var u = data[i1];
+        var v = data[i2];
+        var w = twiddles[stageOffset + j];
+        v.MulMod(w, modulus);
+        var sum = new GpuUInt128(u);
+        sum.AddMod(v, modulus);
+        u.SubMod(v, modulus);
+        data[i1] = sum;
+        data[i2] = u;
+    }
+}