- Add AUDIBLE_ASIN tag to FFmpeg metadata object
- Use -movflags use_metadata_tags to preserve custom tags in mp4/m4b
- Fix .m4b extension detection for mp4 format handling
- Remove redundant -map_metadata 0 option
- Update tests to match new FFmpeg options
The mp4 muxer in FFmpeg only writes standard iTunes tags by default.
Custom tags like AUDIBLE_ASIN are silently dropped unless
use_metadata_tags is specified.
Tested and verified with ffprobe and mediainfo on clean m4b files.