fix(pack): make Reactor.csproj find analyzer DLL at platform-qualified path

Backs out the AppendPlatformToOutputPath=false approach (which broke
unit tests with CS2012 file-lock contention on
Reactor.Localization.Generator/obj/Debug/netstandard2.0/...) in favor
of teaching Reactor.csproj's <None Include> globs about the inherited-
Platform layout.

The cause of the original clean-machine pack failure is unchanged:
when mur pack-local runs `dotnet pack -p:Platform=x64` (or ARM64), the
inherited Platform cascades to the transitive Reactor.Analyzers and
Reactor.Localization.Generator builds, which then land at
bin\<Platform>\<Configuration>\netstandard2.0\. But Reactor.csproj's
pack glob hardcoded bin\<Configuration>\netstandard2.0\ (no Platform
segment), so pack couldn't find the file on a freshly-cloned machine.

Pinning AppendPlatformToOutputPath=false on the analyzers fixed
pack-local but removed bin/obj segregation across Platform values,
which let two concurrent transitive compiles (e.g. dotnet test +
ProjectReference deduplication race) collide on the same
obj\Debug\netstandard2.0\ output path. CI Unit Tests caught this with:

  CSC : error CS2012: Cannot open '...obj\Debug\netstandard2.0\
    Reactor.Localization.Generator.dll' for writing -- The process
    cannot access the file ... because it is being used by another
    process.

New approach: Reactor.csproj computes
$(_ReactorAnalyzerBinDir) = bin\<Configuration>\netstandard2.0
                            when Platform is empty or AnyCPU, OR
                          = bin\<Platform>\<Configuration>\netstandard2.0
                            when Platform is x64 / ARM64.

Pack now finds the analyzers in either layout. The analyzer csprojs
go back to their stock SDK behavior, preserving per-Platform obj/bin
segregation that prevents the build-graph race.

Verified locally:
- mur pack-local from a wiped bin/obj tree succeeds on ARM64 host
  (analyzers land in bin\ARM64\Debug\, Reactor.csproj finds them).
- Plain dotnet build src/Reactor (no -p:Platform) would land
  analyzers at bin\Debug\, also found by the conditional binding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris

src/Reactor.Analyzers/Reactor.Analyzers.csproj

@@ -12,17 +12,6 @@
     <!-- Bundled into Microsoft.UI.Reactor.nupkg — does not ship as its own
          package. See docs/specs/022-packaging-and-distribution.md §5. -->
     <IsPackable>false</IsPackable>
-
-    <!-- Analyzers are platform-independent netstandard2.0 IL. Without this,
-         when Reactor.csproj is packed with `-p:Platform=x64` (or ARM64) the
-         inherited Platform cascades into this transitive build and the
-         output lands at bin\x64\Debug\netstandard2.0\ — but Reactor.csproj's
-         <None Include="...\bin\$(Configuration)\netstandard2.0\..."> for
-         packing the analyzer DLL has no $(Platform) segment and breaks pack
-         on a clean machine (no prior AnyCPU build to fall back to).
-         AppendPlatformToOutputPath=false pins us to bin\$(Configuration)\
-         regardless of inherited Platform. -->
-    <AppendPlatformToOutputPath>false</AppendPlatformToOutputPath>
   </PropertyGroup>
 
   <ItemGroup>

src/Reactor.Localization.Generator/Reactor.Localization.Generator.csproj

@@ -9,12 +9,6 @@
     <IsRoslynComponent>true</IsRoslynComponent>
     <RootNamespace>Microsoft.UI.Reactor.Localization.Generator</RootNamespace>
     <AssemblyName>Reactor.Localization.Generator</AssemblyName>
-
-    <!-- See Reactor.Analyzers.csproj for the full rationale: Reactor.csproj's
-         <None Include="...\bin\$(Configuration)\netstandard2.0\..."> pack
-         glob has no $(Platform) segment, so this transitive source-generator
-         build must not append the inherited Platform to its OutputPath. -->
-    <AppendPlatformToOutputPath>false</AppendPlatformToOutputPath>
   </PropertyGroup>
 
   <ItemGroup>

src/Reactor/Reactor.csproj

@@ -102,10 +102,24 @@
        against the framework itself), so their build outputs must already
        exist before pack — verified by the AssertAnalyzerDllsExist target
        below. -->
+  <!-- Resolve the per-build output dir of the analyzer ProjectReferences.
+       MSBuild propagates $(Platform) to transitive builds: with no
+       -p:Platform (e.g. plain `dotnet build`), the analyzer lands at
+       bin\$(Configuration)\netstandard2.0\; with -p:Platform=x64/ARM64
+       (e.g. `mur pack-local`), it lands at bin\$(Platform)\$(Configuration)\
+       netstandard2.0\ because <AppendPlatformToOutputPath> defaults to
+       true. Compute the path accordingly so pack finds the DLL in either
+       layout — without this, pack-local on a clean machine fails with
+       NU3168/CS2012 because the platform-less path has nothing in it. -->
+  <PropertyGroup>
+    <_ReactorAnalyzerBinDir Condition="'$(Platform)' == '' or '$(Platform)' == 'AnyCPU'">bin\$(Configuration)\netstandard2.0</_ReactorAnalyzerBinDir>
+    <_ReactorAnalyzerBinDir Condition="'$(_ReactorAnalyzerBinDir)' == ''">bin\$(Platform)\$(Configuration)\netstandard2.0</_ReactorAnalyzerBinDir>
+  </PropertyGroup>
+
   <ItemGroup>
-    <None Include="..\Reactor.Analyzers\bin\$(Configuration)\netstandard2.0\Reactor.Analyzers.dll"
+    <None Include="..\Reactor.Analyzers\$(_ReactorAnalyzerBinDir)\Reactor.Analyzers.dll"
           Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
-    <None Include="..\Reactor.Localization.Generator\bin\$(Configuration)\netstandard2.0\Reactor.Localization.Generator.dll"
+    <None Include="..\Reactor.Localization.Generator\$(_ReactorAnalyzerBinDir)\Reactor.Localization.Generator.dll"
           Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
     <None Include="..\..\LICENSE" Pack="true" PackagePath="" Visible="false" />
     <!-- Agent kit packed into the NuGet at `agentkit/`: SKILL.md, every