[feat] add GPU backend support with runtime selection; refactor transcription logic; update CLI and tests

build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    // Only run special build steps when gpu-vulkan feature is enabled.
+    let vulkan_enabled = std::env::var("CARGO_FEATURE_GPU_VULKAN").is_ok();
+    if !vulkan_enabled {
+        return;
+    }
+    // Placeholder: In a full implementation, we would invoke CMake for whisper.cpp with GGML_VULKAN=1.
+    // For now, emit a helpful note. Build will proceed; runtime Vulkan backend returns an explanatory error.
+    println!("cargo:rerun-if-changed=extern/whisper.cpp");
+    println!("cargo:warning=Building with gpu-vulkan: ensure Vulkan SDK/loader are installed. Future versions will compile whisper.cpp via CMake.");
+}