// A runtime media result and a header result are separate observations.
async function inspectTeachingMediaURL(url, timeoutMs = 5000) {
  const response = await fetch(url, {cache:'no-store'});
  const header = {status:response.status, contentType:response.headers.get('content-type'),
    contentLength:response.headers.get('content-length')};
  const media = document.createElement('audio'); media.preload = 'metadata'; media.muted = true;
  const runtime = await new Promise(resolve => {
    let timer;
    const finish = event => {
      clearTimeout(timer); media.onloadedmetadata = media.onerror = null;
      resolve({event, duration:Number.isFinite(media.duration) ? media.duration : null,
        mediaErrorCode:media.error ? media.error.code : null});
    };
    media.onloadedmetadata = () => finish('loadedmetadata');
    media.onerror = () => finish('error');
    timer = setTimeout(() => finish('timeout'), timeoutMs);
    media.src = url; document.body.append(media); media.load();
  });
  media.removeAttribute('src'); media.load(); media.remove();
  return {url, header, runtime, contentListened:false};
}