Checklist

• I'm requesting a feature unrelated to a specific site
• I've looked through the README
• I've verified that I have updated yt-dlp to nightly or master (update instructions)
• I've searched the bugtracker for similar requests including closed ones. DO NOT post duplicates
• I've read the policy against AI/LLM contributions and understand I may be blocked from the repository if it is violated

Provide a description that is worded well enough to be understood

Title: Request: Add AIFF as a supported --audio-format
Body:
Hello yt-dlp developers,
I would like to request adding AIFF as a supported output format for --audio-format when using -x / --extract-audio.
Reasons / Use Case:
AIFF is a lossless format, similar in quality to WAV.
Unlike WAV, AIFF supports embedded thumbnails / cover art, which is very useful for audiobooks, Quran recitations, podcasts, and music.
Many users want a lossless format with metadata support that can preserve album art without converting to MP3 or FLAC.
Suggested Behavior:
When --audio-format aiff is specified, yt-dlp would use ffmpeg to convert the audio to AIFF with PCM encoding.
Embedded thumbnails (if --embed-thumbnail is used) should be preserved in the AIFF file.
Thank you for considering this feature!
It would be very useful for users who need high-quality audio with cover art.

Provide verbose output that clearly demonstrates the problem

• Run your yt-dlp command with -vU flag added (yt-dlp -vU <your command line>)
• If using API, add 'verbose': True to YoutubeDL params instead
• Copy the WHOLE output (starting with [debug] Command-line config) and insert it below

Complete Verbose Output