Better tests and more user feedback while doing transcription; WIP.

src/App.tsx

@@ -59,6 +59,11 @@ export function App() {
   );
   const nextLiveRequest = useRef(0);
 
+  // File-flow run counter (D-018). Cancel bumps it, so a decode that was in
+  // flight when the user cancelled finds itself stale and never posts its
+  // audio to the (fresh) worker. Decoding itself cannot be aborted.
+  const fileRun = useRef(0);
+
   const [busy, setBusy] = useState(false);
   const [decoding, setDecoding] = useState(false);
   const [loadStarted, setLoadStarted] = useState(false);
@@ -213,8 +218,40 @@ export function App() {
     if (loadStarted) startLoad(modelId, preference);
   }
 
+  /**
+   * Cancel the in-flight file transcription (D-018). Transformers.js has no
+   * abort API for a running `asr()` call — and on the WASM backend the pass
+   * blocks the worker's event loop, so even a "cancel" message would sit
+   * unread until the pass finished. Terminating the worker is the only
+   * reliable preemption; it also aborts an in-flight model download. The next
+   * run spawns a fresh worker and reloads the pipeline — cheap, because the
+   * weights stay in the browser Cache API (D-005).
+   */
+  function cancelTranscription() {
+    if (!busy) return;
+    fileRun.current++; // any decode still running is now stale
+    workerRef.current?.terminate();
+    workerRef.current = null;
+    // The file flow never has live passes pending, but if anything is in the
+    // map (defensive), unblock it so nothing awaits a dead worker forever.
+    const cancelled = new Error("Transcription cancelled.");
+    for (const pending of pendingLive.current.values()) pending.reject(cancelled);
+    pendingLive.current.clear();
+    setBusy(false);
+    setPartialText("");
+    setResult(null);
+    setFileName(null);
+    setError(null);
+    // The worker (and any pipeline / partial download) is gone — reset the
+    // load state so the next transcription reloads from a clean slate.
+    setLoadStarted(false);
+    setLoadProgress(initialLoadProgress);
+    setActiveDevice(null);
+  }
+
   async function handleFile(file: File) {
     if (busy || liveStatus !== "idle") return;
+    const run = ++fileRun.current;
     setBusy(true);
     setError(null);
     setPartialText("");
@@ -237,6 +274,7 @@ export function App() {
       // Decoded entirely on-device via Web Audio (D-012); the audio never
       // touches the network (Architecture.md §5).
       const audio = await decodeAudioFile(file);
+      if (fileRun.current !== run) return; // cancelled while decoding (D-018)
       // Transfer the buffer to the worker — zero copy. Language/task only
       // apply to multilingual models; `.en` variants take neither.
       postToWorker(
@@ -247,6 +285,7 @@ export function App() {
         [audio.buffer as ArrayBuffer],
       );
     } catch (decodeError) {
+      if (fileRun.current !== run) return; // this run was already cancelled
       setError(decodeError instanceof Error ? decodeError.message : String(decodeError));
       setBusy(false);
     } finally {