fix(audio_io): decode CAF, and route mp4/m4b paths through ffmpeg

CAF is the container macOS records to by default, but neither the
extension list nor the magic-byte sniffing recognized it, so reads
failed with "Unable to detect audio format from bytes". Detect the caff
magic and send it to ffmpeg alongside the existing MPEG-4/Ogg/WebM
cases, for both path and BytesIO inputs.

Also changed:
- Path reads now route mp4 and m4b to ffmpeg too. Both are ftyp
  containers that the BytesIO path already handled via the header
  sniff, so reading one by path failed where reading the same bytes
  succeeded.
- _detect_format_from_bytes reports caf instead of raising, so callers
  that use it for reporting get a useful format name.

Adds CAF read coverage for both path and BytesIO inputs.