ci: cap Windows test build at --parallel 2 — vcpkg z-applocal DLL-copy race in the shared output dir (run 28949911161)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019btwFxDt7D19SZSqPYvZnk

Author: secup

.github/workflows/build-matrix.yml

@@ -144,7 +144,12 @@ jobs:
 
       - name: Build Tests (Windows)
         if: runner.os == 'Windows'
-        run: cmake --build build --config Debug --parallel
+        # --parallel 2: vcpkg's z-applocal post-build step copies the SAME
+        # runtime DLLs into the shared tests/Debug output dir for every test
+        # target — at full parallelism two copies collide ("file is being used
+        # by another process", run 28949911161). Two concurrent post-builds
+        # make the race effectively improbable; same bound as Linux.
+        run: cmake --build build --config Debug --parallel 2
 
       - name: Run CTest (Linux/macOS)
         if: runner.os != 'Windows'