fix: default distributions to automatic GPU support with CPU fallback

docs/BUILDING.md

@@ -86,6 +86,14 @@ search starts a model process; the desktop retains a prewarmed query worker.
 
 ## Cross-compilation and native release matrix
 
+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.
+The compilation SDK is not an end-user requirement. Explicit `--backend cpu`
+commands in the VM development sections are internal test configurations.
+
 The shared Rust engine/service has no GUI dependency. Install a destination
 Rust target with `rustup target add TARGET`, then build using
 `cargo build -p searchi-cli --target TARGET --locked`. This also requires the