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>

scripts/ci/ensure-windows-toolchain.ps1

@@ -0,0 +1,21 @@
+# Idempotent pre-flight for punktfunk's Windows CI dependencies: WDK + cargo-wdk (driver builds),
+# FFmpeg x64/ARM64 trees, Inno Setup, and the aarch64-pc-windows-msvc rustup target. Run at the
+# start of every Windows CI job so ANY runner - freshly built from unom/infra's windows-runner/
+# template, rebuilt, or a new one added later - self-provisions on first real use, instead of
+# needing a human to remember to dispatch a separate provisioning workflow first (and instead of
+# racing which runner a manually-dispatched provisioning workflow happens to land on, when more
+# than one Windows runner shares the windows-amd64 label).
+#
+# Each underlying script already does its own existence checks (Test-Path/Get-Command) before
+# installing anything, so this is a fast no-op on a runner that's already fully provisioned - the
+# only cost is on a genuinely fresh box, where it's the first job's problem to pay once.
+$ErrorActionPreference = "Stop"
+trap {
+  Write-Host "FATAL: $_"
+  Write-Host $_.ScriptStackTrace
+  exit 1
+}
+
+$ciDir = $PSScriptRoot
+& "$ciDir\provision-windows-wdk.ps1"
+& "$ciDir\provision-windows-punktfunk-extras.ps1"