ci(windows-drivers): LLVM via portable tar.xz + self-provision driver-build

The LLVM NSIS .exe /S silent install HANGS in the headless SYSTEM CI session
(stuck >15min after download, blocking the single runner). Switch to the portable
clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar.xz (curl + Win11 tar -xf, strip 1) —
deterministic, no installer. And make driver-build run the provision script itself
(idempotent) so it self-provisions LLVM and never races a separate provision run.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/windows-drivers.yml

@@ -124,6 +124,11 @@ jobs:
       LIBCLANG_PATH: 'C:\llvm-21\bin'
     steps:
       - uses: actions/checkout@v4
+      - name: Ensure WDK + cargo-wdk + LLVM 21.1.2 (idempotent self-provision)
+        # Run the provisioning script here too so driver-build is self-sufficient and never races a
+        # separate provision run on the single runner. Path is relative to the job working-directory
+        # (packaging/windows/drivers). Near-noop once the toolchain is present.
+        run: ../../../scripts/ci/provision-windows-wdk.ps1
       - name: cargo build wdk-probe (windows-drivers-rs)
         run: cargo build -p wdk-probe -v
       - name: Inspect the produced DLL's /INTEGRITYCHECK bit