Refactor SPMD jobs for true vectorized/masked execution

- Change IJobSPMD.Execute to (indices, mask, ctx) signature for all arities, enabling proper vectorized/masked SPMD execution.
- Update all SPMD job wrappers, extension methods, and test jobs to use new interface.
- Add AVX2 gather/masked gather support to MathV.GatherVector2/3/4 and related methods; use [ConstantExpected] byte scale.
- Improve gather/select logic, pointer arithmetic, and overloads for ref/int* index access.
- Refactor GGXMipGenerationBenchmark and jobs for SPMD, with per-mip-level vectorized jobs and improved memory access.
- Clean up code, fix naming, update comments, and bump version to 1.3.6.

Misaki.HighPerformance.Mathematics.SPMD/Misaki.HighPerformance.Mathematics.SPMD.csproj

@@ -7,7 +7,7 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <Authors>Misaki</Authors>
-    <AssemblyVersion>1.3.5</AssemblyVersion>
+    <AssemblyVersion>1.3.6</AssemblyVersion>
     <Version>$(AssemblyVersion)</Version>
     <PackageProjectUrl>https://git.personalnas.com/Misaki/Misaki.HighPerformance.git</PackageProjectUrl>
     <RepositoryUrl>https://git.personalnas.com/Misaki/Misaki.HighPerformance.git</RepositoryUrl>
@@ -22,7 +22,7 @@
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
     <IsAotCompatible>True</IsAotCompatible>
-    <DefineConstants>$(DefineConstants)</DefineConstants>
+    <DefineConstants>$(DefineConstants);MHP_FASTMATH</DefineConstants>
   </PropertyGroup>
   
   <ItemGroup>