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/drivers/pf-xusb/README.md

@@ -66,8 +66,8 @@ there); these repo files are the canonical copies — keep them in sync.
 `crates/punktfunk-host/src/inject/gamepad_windows.rs` is the Windows `GamepadManager` (used by
 `PadBackend::Xbox360`): it SwDeviceCreate's the `pf_xusb` companion, maps `pfxusb-shm-<index>`, writes
 the XInput state from the client's gamepad frame (already XInput-convention) and forwards rumble. There
-is **no ViGEmBus dependency** anymore. The driver is vendored + pnputil-installed by the Inno Setup
-installer (`packaging/windows/gamepad-drivers/` + `install-gamepad-drivers.ps1`).
+is **no ViGEmBus dependency** anymore. The driver is built from source (`packaging/windows/drivers/pf-xusb`),
+signed, and pnputil-installed by the Inno Setup installer (via `install-gamepad-drivers.ps1`).
 
 ## Multi-pad