Cobalt 11.7.1 origin muxer repair

Upstream: https://github.com/imputnet/cobalt
Upstream revision: a636575b09de1fc55d9b8cd98cac88f5f2f16b42
Upstream source files: api/src/stream/ffmpeg.js and api/src/stream/internal-hls.js
Upstream license: GNU Affero General Public License version 3 or later (LICENSE).
Copyright remains with imput and the upstream contributors.

Modified 2026-11-08 by Bento maintainers under the same license:
- A single-input video remux accepts an absent audio stream; video remains required.
- Known FFmpeg output containers receive the corresponding HTTP Content-Type.
- Complete the existing bounded length estimate before starting FFmpeg, attach the
  output pipe without another asynchronous gap, and avoid starting work for a
  response closed during estimation.
- Parsed and rewritten HLS playlists receive application/vnd.apple.mpegurl so
  extensionless internal HTTP URLs are recognized without relaxing FFmpeg checks.
- A failed bounded size estimate no longer rejects otherwise valid media; unknown
  and nonpositive estimates omit the advisory header. Streaming limits remain.
- Audio conversion, authentication, URLs, budgets and runtime limits are unchanged.

This directory contains public upstream material and a generic vendor repair only.
It contains no private Bento application code, deployment configuration or keys.
The corresponding-source bundle must remain publicly available, with a clear
source offer to remote users, before this modified server is deployed publicly.
See README.md for the complete-source packaging and standalone build procedure.
