fix: restore DiagnosticClasses polyfill, normalize import paths on Windows

Restore accidentally removed InjectDiagnosticClassesOnLegacy ItemGroup.
Normalize MakeRelative backslashes to forward slashes in ClaudeBrain
@import paths for cross-platform consistency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ANcpLua

src/Build/Common/Common.targets

@@ -266,7 +266,7 @@
   <Target Name="GenerateClaudeMd" BeforeTargets="Build" Condition="'$(GenerateClaudeMd)' == 'true' AND !Exists('$(MSBuildProjectDirectory)/CLAUDE.md')">
     <PropertyGroup>
       <_RootClaudeMd>$([MSBuild]::GetPathOfFileAbove('CLAUDE.md', '$(MSBuildProjectDirectory)'))</_RootClaudeMd>
-      <_RootClaudeMdRelative Condition="'$(_RootClaudeMd)' != ''">$([MSBuild]::MakeRelative('$(MSBuildProjectDirectory)', '$(_RootClaudeMd)'))</_RootClaudeMdRelative>
+      <_RootClaudeMdRelative Condition="'$(_RootClaudeMd)' != ''">$([MSBuild]::MakeRelative('$(MSBuildProjectDirectory)', '$(_RootClaudeMd)').Replace('\','/'))</_RootClaudeMdRelative>
     </PropertyGroup>
 
     <!-- SDK variant detection -->

src/Build/Common/LegacySupport.targets

@@ -177,6 +177,10 @@
        DIAGNOSTICS (< net6.0)
        StackTraceHiddenAttribute
        ───────────────────────────────────────────────────────────────────────── -->
+  <ItemGroup Condition="'$(InjectDiagnosticClassesOnLegacy)' == 'true' AND ('$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == 'net472')">
+    <Compile Include="$(_SharedSourcePath)Polyfills/DiagnosticClasses.cs" LinkBase="Shared/Polyfills" InProject="false"/>
+  </ItemGroup>
+
   <ItemGroup Condition="('$(InjectStackTraceHiddenOnLegacy)' == 'true' OR '$(InjectANcpLuaDiagnosticsPolyfills)' == 'true') AND '$(_NeedsNet6Polyfills)' == 'true'">
     <Compile Include="$(_SharedSourcePath)Polyfills/Diagnostics/StackTraceHiddenAttribute.cs" LinkBase="Shared/Polyfills/Diagnostics" InProject="false"/>
   </ItemGroup>