Render and merge candidates

const history = await api.get(`/api/editing/history/${encodeURIComponent(songId)}`);
const project = await api.get(`/api/songs/${encodeURIComponent(songId)}`);
const candidates = history.takes.filter(take => selectedTakeIds.includes(take.id));
if (!candidates.length) throw new Error('Select pending candidates to merge');
const region = { start: 15, end: 30 };
const placements = candidates.map(take => ({ takeId: take.id, shift: 0 }));
const original = true; // Include the current master beneath the candidates.

const form = new FormData();
form.append('file', wav, 'composition.wav');
const { url } = await api.request('/api/editing/source?' +
  new URLSearchParams({ songId }), { method: 'POST', form });

const merged = await api.post('/api/editing/activate', {
  songId, userAddress,
  takeId: candidates[0].id,
  action: 'merge',
  composition: {
    takeIds: candidates.map(take => take.id),
    placements, region, original,
    commitId: history.commits[0]?.id ?? null,
    srcAudioUrl: url,
  },
});