Refactor SIMD gather, tighten constraints, doc & test opts

- Require TLane : unmanaged, ISPMDLane for stricter type safety and direct memory ops
- Refactor GatherVectorN and WideLane<T>.Gather to use Unsafe.SkipInit and direct assignment, removing stackalloc and TLane.Load for better SIMD performance
- Use Vector.Sum in WideLane<T>.ReduceAdd
- Add/improve XML docs for ReduceAdd/ReduceMax/ReduceMin
- Update test project for AOT, AVX2, speed optimization, and disable reflection
- Tweak GGXMipGenerationBenchmark and Program.cs for improved benchmarking and output

Misaki.HighPerformance.Mathematics.SPMD/ISPMDLane.cs

@@ -597,13 +597,22 @@ public unsafe interface ISPMDLane<TSelf, TNumber> : ISPMDLane, IEquatable<TSelf>
     static abstract TSelf Rsqrt(TSelf value);
 
     /// <summary>
-    /// Horizontally reduces the lane value by adding all lanes together, returning a single-lane result.
+    /// Reduces the lane value to a single scalar by adding all lanes together.
     /// </summary>
-    /// <param name="a"></param>
-    /// <param name="b"></param>
-    /// <returns></returns>
+    /// <param name="value">The lane value to reduce.</param>
+    /// <returns>The reduced scalar value.</returns>
     static abstract TNumber ReduceAdd(TSelf value);
+    /// <summary>
+    /// Reduces the lane value to a single scalar by finding the maximum element.
+    /// </summary>
+    /// <param name="value">The lane value to reduce.</param>
+    /// <returns>The reduced scalar value.</returns>
     static abstract TNumber ReduceMax(TSelf value);
+    /// <summary>
+    /// Reduces the lane value to a single scalar by finding the minimum element.
+    /// </summary>
+    /// <param name="value">The lane value to reduce.</param>
+    /// <returns>The reduced scalar value.</returns>
     static abstract TNumber ReduceMin(TSelf value);
 
     /// <summary>