Ship the OpenVINO and WebGPU runtimes in every desktop package

The Windows installer and the Flatpak carried no ONNX Runtime, so they
ran every model on tract's one core; the Arch package left it to an
optional dependency. Each now installs two builds under runtimes/ —
Intel's OpenVINO build and the generic WebGPU one, both with the CPU
provider — fetched by tools/fetch-bundled-runtimes.sh from PyPI wheels
pinned by SHA-256, pruned to the native libraries (81 + 31 MB on Linux,
67 + 42 MB on Windows), licence texts beside them.

darkroom-desktop searches runtimes/openvino and runtimes/webgpu under
each place a package installs to; the engine opens all it finds and
keeps the one that fits the GPU, so a CUDA or ROCm runtime installed
beside them still wins on its vendor's card. On Windows the chosen
runtime's directory goes on PATH, because Intel's build leaves OpenVINO's
DLLs for the loader to find there.

The Windows image gains unzip; the installer smoke test checks both
runtimes landed.

packaging/windows/darkroom.nsi

@@ -84,6 +84,12 @@ Section "DarkRoom" SecMain
   SetOutPath "$INSTDIR\manual"
   File /r "${STAGE}\manual\*"
 
+  ; ONNX Runtime, twice: Intel's OpenVINO build and the WebGPU build. The
+  ; application opens both and keeps the one that fits the GPU
+  ; (docs/dev/inference.md §3.2); each also runs the models on every core.
+  SetOutPath "$INSTDIR\runtimes"
+  File /r "${STAGE}\runtimes\*"
+
   WriteUninstaller "$INSTDIR\uninstall.exe"
 
   ; Add/Remove Programs. HKCU, to match the per-user install.
@@ -123,6 +129,7 @@ Section "Uninstall"
   Delete "$INSTDIR\uninstall.exe"
   RMDir /r "$INSTDIR\models"
   RMDir /r "$INSTDIR\manual"
+  RMDir /r "$INSTDIR\runtimes"
   RMDir "$INSTDIR"
 
   Delete "$SMPROGRAMS\${NAME}\${NAME}.lnk"