[CI] Collect an MSBuild performance summary and binary log on request

The Windows job spends about 28 minutes inside one "Build project"
group, with nothing in it that says where the time goes: MSBuild runs
single-node because no --parallel is passed, so projects build one
after another and it is not visible which projects, tasks or phases are
responsible.

Add a --profile flag that makes the Windows build write
-flp:PerformanceSummary and -bl, repeat the summary in the job log so
the numbers need no download, and report the core count, which decides
how much parallelism is worth asking for. The binary log is uploaded as
an artifact only on the runs that asked for it.

The workflow drives it from a build_profile matrix key that is empty by
default, so no build changes behaviour until someone sets it.

.github/workflows/desktop-build.yml

@@ -337,6 +337,10 @@ jobs:
             target: 10
             runner: windows-2025    # https://github.com/actions/runner-images/blob/main/images/windows/Windows2025-VS2026-Readme.md
 
+            # Set build_profile: 1 to have the build write an MSBuild performance
+            # summary and a binary log; empty on every entry that runs by default.
+            build_profile: ''
+
             cmake_generator: "Visual Studio 18 2026"
             cmake_generator_platform: x64
             make_package: 1
@@ -352,6 +356,7 @@ jobs:
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 100
     env:
+      BUILD_PROFILE: '${{ matrix.build_profile }}'
       CCACHE_DIR: ${{ github.workspace }}/.cache/
       CCACHE_SIZE: 600M    # space of all repo is 10Gi: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy
 
@@ -653,6 +658,17 @@ jobs:
             build/oracle/Release/*.pdb
             build/servatrice/Release/*.pdb
 
+      # Written only when the matrix entry sets build_profile, so the artifact
+      # exists exactly on the runs that were asked to produce one.
+      - name: "[Windows] Upload MSBuild binary log"
+        if: always() && matrix.os == 'Windows' && env.BUILD_PROFILE == '1'
+        uses: actions/upload-artifact@v7
+        with:
+          if-no-files-found: error
+          name: ${{ steps.build.outputs.name }}-msbuild
+          path: build/msbuild.binlog
+          retention-days: 7
+
       - name: "Upload to release"
         if: needs.configure.outputs.tag != null && matrix.make_package == '1'
         id: upload_release