Feat: clean the codebase as much as possible

src/main.rs

@@ -149,7 +149,7 @@ fn main() -> io::Result<()> {
                         std::thread::Builder::new()
                             .name("sample-preload".into())
                             .spawn(move || {
-                                init::preload_sample_heads(preload_entries, sr, &registry);
+                                engine::preload_sample_heads(preload_entries, sr, &registry);
                             })
                             .expect("failed to spawn preload thread");
                     }