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/punktfunk-host.iss

@@ -40,7 +40,7 @@
 #ifdef StageDir
   #define WithDriver
 #endif
-; GamepadStageDir (the vendored UMDF gamepad drivers + install-gamepad-drivers.ps1) is optional.
+; GamepadStageDir (the built-from-source UMDF gamepad drivers + install-gamepad-drivers.ps1) is optional.
 #ifdef GamepadStageDir
   #define WithGamepad
 #endif
@@ -126,7 +126,7 @@ Source: "{#WebSetup}"; DestDir: "{tmp}"; DestName: "web-setup.ps1"; Flags: delet
 Source: "{#StageDir}\*"; DestDir: "{tmp}\pfvdisplay"; Flags: deleteafterinstall recursesubdirs createallsubdirs; Tasks: installdriver
 #endif
 #ifdef WithGamepad
-; The vendored UMDF gamepad drivers + install-gamepad-drivers.ps1, extracted to {tmp}, removed after.
+; The built-from-source UMDF gamepad drivers + install-gamepad-drivers.ps1, extracted to {tmp}, removed after.
 Source: "{#GamepadStageDir}\*"; DestDir: "{tmp}\gamepad"; Flags: deleteafterinstall recursesubdirs createallsubdirs; Tasks: installgamepad
 #endif
 #ifdef WithVkLayer