/* Teaching probe for the supplied loopback-only generated-tone fixture.
   Never requests microphone, calls play(), or reads user files. */
async function probeTeachingPreload(mode, fetchSeparately = false) {
  if (location.hostname !== '127.0.0.1') throw new Error('Use only the supplied local fixture');
  if (!['none', 'metadata', 'auto', 'deferred-source'].includes(mode)) throw new Error('Unknown teaching preload mode');
  const audio = document.createElement('audio');
  audio.muted = true;
  audio.preload = mode === 'deferred-source' ? 'none' : mode;
  if (mode !== 'deferred-source') audio.src = '/tone.wav';
  document.body.append(audio);
  if (mode !== 'deferred-source') audio.load();
  let separateFetchBytes = null;
  if (fetchSeparately) {
    const response = await fetch('/tone.wav', {cache: 'no-store'});
    if (!response.ok) throw new Error('Local teaching audio unavailable');
    separateFetchBytes = (await response.arrayBuffer()).byteLength;
  }
  await new Promise(resolve => setTimeout(resolve, 1200));
  const snapshot = {
    requestedMode: mode,
    sourceAssigned: mode !== 'deferred-source',
    loadCalledBeforeObservation: mode !== 'deferred-source',
    reflectedPreload: audio.preload,
    readyState: audio.readyState,
    networkState: audio.networkState,
    finiteDuration: Number.isFinite(audio.duration) ? audio.duration : null,
    paused: audio.paused,
    currentTime: audio.currentTime,
    separateFetchBytes,
    observationWindowMs: 1200,
    playbackPerformed: false
  };
  audio.removeAttribute('src');
  audio.load();
  audio.remove();
  return snapshot;
}