build: package universal Windows runtime and verify automatic CPU fallback

docs/BUILDING.md

@@ -89,8 +89,11 @@ search starts a model process; the desktop retains a prewarmed query worker.
 Normal distribution worker builds and staging default to `auto`: Metal on
 macOS, Vulkan on Windows/Linux, with CPU fallback in the same application.
 Users do not choose a CPU/GPU edition or rebuild for their graphics card.
-Windows/Linux build machines need the Vulkan SDK including `glslc`; absence
-fails release preparation instead of quietly generating a CPU-only runtime.
+Windows/Linux build machines need the Vulkan SDK including `glslc`. Windows
+x64 provisioning now downloads the pinned, checksum-verified official SDK into
+the workspace using copy-only mode; it does not install drivers or change the
+system environment. Linux builders provision their distro's Vulkan development
+tools. Missing required tools fail release preparation rather than omitting GPU.
 The compilation SDK is not an end-user requirement. Explicit `--backend cpu`
 commands in the VM development sections are internal test configurations.
 
@@ -133,7 +136,9 @@ matching-architecture Google worker, FFmpeg/FFprobe and notices. It excludes
 model caches, local settings and indexes. `PACKAGE.json` records SHA-256 for
 each included file. Extract the entire archive before launching `searchi.exe`.
 First launch uses normal verified model setup. WebView2 and the applicable
-Microsoft C++ runtime remain Windows prerequisites.
+Microsoft C++ runtime remain Windows prerequisites. The Windows archive includes
+the verified Vulkan loader DLL and its license; GPU availability is detected at
+runtime, with CPU fallback. The SDK is used only to compile the app.
 
 This is an unsigned development archive, not a production installer. Native
 launch, clean-machine dependencies, upgrades and signatures must still be