Secondary audio tracks are dropped and channel layout is forced to stereo

Steps to reproduce

1. Create a source with two audio tracks (mimics OBS mic + desktop):
   ffmpeg -f lavfi -i testsrc2=size=640x360:rate=30
   -f lavfi -i sine=frequency=440 -f lavfi -i sine=frequency=880
   -t 2 -map 0:v -map 1:a -map 2:a
   -c:v libx264 -pix_fmt yuv420p -c:a aac -y /tmp/multi.mp4

2. ./resolve_convert.sh -o /tmp/out /tmp

3. ffprobe -v error -show_entries stream=index,codec_type,codec_name,channels
   -of csv=p=0 /tmp/out/multi.mov

Expected behavior

Both audio tracks preserved, original channel layouts intact.

Actual behavior

SOURCE:  0,h264,video / 1,aac,audio,1 / 2,aac,audio,1
OUTPUT:  0,dnxhd,video / 1,pcm_s24le,audio,2

The second track is gone, and -ac 2 upmixed the surviving mono track to
fake stereo.

Two causes:

• No -map, so ffmpeg's default selection takes one video and one audio
  stream and discards the rest.
• -ac 2 forces a stereo layout regardless of the source, downmixing
  surround and upmixing mono.

This affects anyone recording separate mic and desktop tracks in OBS, and
anyone with multi-channel source audio. The loss is silent — it surfaces
in Resolve after the footage is already on the timeline.

Environment

OS: Nobara 44
ffmpeg: 8.1.2
bash: 5.3.9(1)-release (x86_64-redhat-linux-gnu)

Start with resolve_convert.sh and reproduce the issue using the provided ffmpeg command and conversion command. Use ffprobe on the output to verify that both audio tracks are preserved and their original channel layouts remain intact; the issue is done when the output matches those expectations.