feat(arr): name subtitle codecs and dispositions

SubtitleTrack now carries the codec, split text formats from bitmap
ones per DESIGN.md §15, plus the forced and SDH dispositions ffprobe
reports. Without the forced flag a file carrying only a forced track
read as satisfied for that language.

scripts/make-probe-fixtures.sh

@@ -19,19 +19,21 @@ trap 'rm -rf "$work"' EXIT
 
 mkdir -p "$out"
 
-# 1080p feature: three audio tracks covering all three §5.2 signals, plus two
-# subtitle tracks. Three seconds, so the runtime sanity check has something to
+# 1080p feature: three audio tracks covering all three §5.2 signals, plus
+# three subtitle tracks — two plain text, one carrying §15's forced
+# disposition. Three seconds, so the runtime sanity check has something to
 # compare against.
 printf '1\n00:00:00,000 --> 00:00:01,000\nhello\n\n' > "$work/en.srt"
 printf '1\n00:00:00,000 --> 00:00:01,000\nolá\n\n' > "$work/pt.srt"
+printf '1\n00:00:00,500 --> 00:00:01,500\nSIGN\n\n' > "$work/forced.srt"
 
 ffmpeg -hide_banner -loglevel error -y \
     -f lavfi -i "color=c=black:s=1920x1080:r=24:d=3" \
     -f lavfi -i "sine=frequency=440:duration=3" \
     -f lavfi -i "sine=frequency=660:duration=3" \
     -f lavfi -i "sine=frequency=880:duration=3" \
-    -i "$work/en.srt" -i "$work/pt.srt" \
-    -map 0:v -map 1:a -map 2:a -map 3:a -map 4:s -map 5:s \
+    -i "$work/en.srt" -i "$work/pt.srt" -i "$work/forced.srt" \
+    -map 0:v -map 1:a -map 2:a -map 3:a -map 4:s -map 5:s -map 6:s \
     -c:v libx264 -preset ultrafast -crf 51 -pix_fmt yuv420p \
     -c:a aac -b:a 8k -c:s srt \
     -metadata:s:a:0 language=eng -metadata:s:a:0 title="English" \
@@ -39,6 +41,7 @@ ffmpeg -hide_banner -loglevel error -y \
     -metadata:s:a:1 handler_name="Portuguese (Brazil)" \
     -metadata:s:a:2 language=pt-PT \
     -metadata:s:s:0 language=eng -metadata:s:s:1 language=por \
+    -metadata:s:s:2 language=eng -disposition:s:2 forced \
     "$out/movie-1080p.mkv"
 
 # The extras reel trap: larger on disk than the feature (noise compresses