Scaffold WhiteMagic solution and add AGENTS.md

Create the WhiteMagic net8.0-windows class library and the WhiteMagicTest
xUnit project, grouped in WhiteMagic.slnx (SDK 10 default format). Library
enables nullable, unsafe blocks, x86/x64 platforms, warnings-as-errors.
Empty solution builds clean (0 errors, 0 warnings).

Add AGENTS.md (ASD-STE100) defining the build/test commands, the test-first
rule, and the one-feature-one-branch workflow with review before merge to
master.

Mark project-setup tasks 1.1-1.3, 1.5 done; 1.4 (Native P/Invoke surface)
is the first feature branch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

WhiteMagic/WhiteMagic.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0-windows</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <Platforms>x86;x64;AnyCPU</Platforms>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
+</Project>