HLS muxing variant streams has some incorrect behaviour

[vckt]

Summary of the bug:
The HLS muxer has some weird behaviour that I think is incorrect, such as producing all variant streams with default=yes, when only one variant stream in group is allowed to have that, lack of autoselect.
I am a beginner and have made a patch, so I would appreciate some guidance on getting it accepted.
How to reproduce:

% ffmpeg -i input -f hls -map 0:v:0 -map 0:a:0 -map 0:a:0 -b:v:0 1000k -b:a:0 200k -b:a:1 100k -var_stream_map "v:0,agroup:g1 a:0,agroup:g1 a:1,agroup:g1 out-%v.ts
ffmpeg N-116133-g03175b587c
built with gcc 14.1.0

This will produce the following:

#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_g1",NAME="audio_1",DEFAULT=YES,URI="out-1.ts"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="group_g1",NAME="audio_2",DEFAULT=YES,URI="out-2.ts"

However when a default is specified, DEFAULT=NO is printed for other variant streams, but this is not necessary as in absence of default it is assumed to be NO.

[vckt]

Reference: RFC8216 4.3.4.1