Key browser preview caches on a file generation, and hash bytes only when that key misses.

Replacing a virtual file bumps a counter the UI lookup reads. The content hash runs on that miss, from a callback, so the frame path does not stat every footage item or hash on every store write.

apps/effectcraft-web/src/app.rs

@@ -257,6 +257,7 @@ pub async fn start(canvas_id: String) -> Result<(), JsValue> {
                 if n > 0 {
                     app.frames.set_remote(Some(Arc::new(frames::WebFrames::start(n, !query_flag("nogpuworkers")))));
                 }
+                app.frames.set_virtual_media(files::STORE.media_epoch(), Arc::new(|path| files::STORE.file_identity(path)));
                 let (tx, rx) = std::sync::mpsc::channel();
                 api::set_sender(tx);
                 app = app.with_control(rx);