[feat] implement backend abstraction, dynamic backend selection, and GPU feature integration

crates/polyscribe-core/build.rs

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: MIT
+// Move original build.rs behavior into core crate
+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;
+    }
+    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."
+    );
+}