fix(ci): pass -p:Platform=$hostArch to TestApp build

Scaffolded apps inherit Directory.Build.props:
  <WindowsAppSDKSelfContained>true</WindowsAppSDKSelfContained>

The SelfContained target rejects AnyCPU (it needs a concrete arch to
embed the right runtime), so `dotnet build TestApp` with no `-p:Platform`
fails on a fresh clone:

  Microsoft.WindowsAppSDK.SelfContained.targets(74,9): error :
    WindowsAppSDKSelfContained requires a supported Windows architecture.

Detect $env:PROCESSOR_ARCHITECTURE and pass it through. This is the
same wart the user flagged earlier ("we don't want to REQUIRE devs to
do self contained") — addressing that properly means changing the
default in Directory.Build.props or in the template, which is bigger
than this PR's scope. For now, unblock CI.

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

Author: codemonkeychris

.github/workflows/bootstrap.yml

@@ -164,8 +164,14 @@ jobs:
       - name: Build TestApp (default WindowsAppSDKSelfContained=true)
         shell: pwsh
         run: |
+          # WindowsAppSDKSelfContained=true (inherited from
+          # Directory.Build.props) requires a concrete arch to embed the
+          # runtime under — AnyCPU is rejected by the SelfContained
+          # target. Pass the host arch explicitly.
+          $arch = if ($env:PROCESSOR_ARCHITECTURE -eq 'ARM64') { 'ARM64' } else { 'x64' }
           dotnet build TestProjects/TestApp/TestApp.csproj `
               -c Release `
+              "-p:Platform=$arch" `
               --nologo -v:m
           if ($LASTEXITCODE -ne 0) { throw "TestApp build exited $LASTEXITCODE" }