fix(probe): do not hide prober failures or cover art

Review on #59. select_feature skipped every probe error, so a missing
ffprobe or a timeout reported "no video file" for a torrent that has
one; only per-file content failures are skipped now. And an audio file
with embedded artwork reports the cover as a video stream, which made a
soundtrack look importable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/make-probe-fixtures.sh

@@ -144,6 +144,15 @@ PY
 python3 "$work/dovi.py" "$work/base.mp4" "$out/dv-profile5.mp4" 5 0
 python3 "$work/dovi.py" "$work/base.mp4" "$out/dv-profile8-1.mp4" 8 1
 
+# An audio file with embedded artwork. ffprobe reports the cover as a video
+# stream, so this is what stops a soundtrack passing for a feature.
+ffmpeg -hide_banner -loglevel error -y \
+    -f lavfi -i "color=c=red:s=64x64:d=1" -frames:v 1 "$work/cover.jpg"
+ffmpeg -hide_banner -loglevel error -y \
+    -f lavfi -i "sine=frequency=440:duration=2" -i "$work/cover.jpg" \
+    -map 0:a -map 1:v -c:a aac -b:a 8k -c:v mjpeg -disposition:v attached_pic \
+    "$out/audio-with-cover.m4a"
+
 # Not media at all: the thing a multi-file torrent is full of.
 printf 'Release notes. Not a video file.\n' > "$out/notes.nfo"