chore(windows): clean up build/packaging - drop vendored driver binaries + the LLVM-21 pin

Now that the drivers build from source in CI, remove the dead checked-in binaries and
the toolchain cruft they left behind:

- Delete packaging/windows/{pf-vdisplay,gamepad-drivers}/ (the prebuilt .dll/.inf/.cat/.cer).
  pack-host-installer.ps1 builds + signs all three drivers from the drivers/ workspace and
  nothing reads the vendored dirs anymore; stage-pf-vdisplay.ps1's -VendorDir is now a
  mandatory build-output path, not a vendored default.
- Drop the LLVM-21 pin. The vendored bindgen 0.71->0.72 bump (the shipping pack already
  builds green on the runner-default clang 22) retired the bindgen-0.71 layout-test overflow
  that needed LLVM 21.1.2, so windows-drivers.yml + provision-windows-wdk.ps1 no longer
  install/point at C:\llvm-21 (~898 MB off a fresh provision) - both driver builds now use one
  toolchain (clang 22 + bindgen 0.72).
- pack -SkipBuild on the gamepad build (build-pf-vdisplay.ps1 already builds the whole
  workspace), build-web.ps1 reaps a stale node too, deploy-dev.ps1 nefconc path + comments.
- Reword the vendored-driver references (build scripts, .iss, READMEs, the vite web-bundle
  comment) to the build-from-source reality.

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

packaging/windows/pack-host-installer.ps1

@@ -164,7 +164,9 @@ else { Write-Host "-NoDriver: building installer WITHOUT the bundled pf-vdisplay
 # gamepad-drivers.ps1 adds each to the store (the host SwDeviceCreate's the per-session devnodes).
 if (-not $NoDriver) {
     $gpBuilt = Join-Path $OutDir 'gamepad-built'
-    & (Join-Path $here 'build-gamepad-drivers.ps1') -Out $gpBuilt
+    # -SkipBuild: build-pf-vdisplay.ps1 above already `cargo build`s the WHOLE drivers workspace (incl.
+    # the gamepad cdylibs), so just sign+stage them here - no redundant second full build.
+    & (Join-Path $here 'build-gamepad-drivers.ps1') -Out $gpBuilt -SkipBuild
     $gpStage = Join-Path $OutDir 'gamepad'
     if (Test-Path $gpStage) { Remove-Item -Recurse -Force $gpStage }
     New-Item -ItemType Directory -Force -Path $gpStage | Out-Null