feat(windows-ci): self-provisioning toolchain for the shared unom/infra runner

The Windows CI runner (home-windows-runner-1, vmid 210) is now provisioned/owned by
unom/infra and can be rebuilt or joined by additional windows-amd64-labeled runners at
any time - a manually-dispatched provisioning workflow has no way to target a specific
runner instance, so it could land on an already-provisioned box instead of the one that
needed it. Replace windows-drivers-provision.yml / windows-punktfunk-provision.yml with
scripts/ci/ensure-windows-toolchain.ps1, a shared idempotent pre-flight (WDK/cargo-wdk,
FFmpeg, Inno Setup, ARM64 rustup target) that every Windows workflow now runs at job
start - a fast no-op once already provisioned, so any runner self-heals on first real use.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

design/windows-host-rewrite.md

@@ -250,9 +250,10 @@ then restore + `git worktree remove`. Drive over ssh via `powershell -EncodedCom
 The persistent build validator is the **windows-amd64 CI runner** (no GPU — fine for builds / `iddcx`
 link / `/INTEGRITYCHECK` self-sign / the surface-asserts; live NVENC encode + on-glass defers to the RTX
 box). Workflows: `windows-host.yml` (the host installer), `windows-drivers.yml` (the driver workspace
-build + FORCE_INTEGRITY clear), `windows-drivers-provision.yml` (WDK/LLVM toolchain), `windows-msix.yml`
-(the client). A single Windows runner serializes the whole fleet; a `Cargo.toml` touch costs ~25 min of
-queue, so driver pushes that avoid `Cargo.toml` skip the fleet serialization.
+build + FORCE_INTEGRITY clear; self-provisions the WDK/LLVM toolchain via `scripts/ci/
+ensure-windows-toolchain.ps1`), `windows-msix.yml` (the client). A single Windows runner serializes
+the whole fleet; a `Cargo.toml` touch costs ~25 min of queue, so driver pushes that avoid
+`Cargo.toml` skip the fleet serialization.
 
 Local pre-push checks (this Linux box can't compile the Windows paths):
 ```sh