feat(api): extract an embedded track on demand (#261)

Closes #260.

crates/arr-core/src/lib.rs

@@ -364,6 +364,23 @@ impl SubtitleCodec {
     pub const fn is_text(self) -> bool {
         matches!(self, Self::SubRip | Self::Ass | Self::MovText)
     }
+
+    /// The inverse of [`Display`](fmt::Display): read back a codec that was
+    /// written out under its `ffprobe` name. `ffprobe`'s own aliases are
+    /// accepted alongside, so a column written by an older probe still
+    /// resolves. Anything unrecognised is [`Self::Other`], which extracts
+    /// nothing — the same answer as a bitmap track.
+    #[must_use]
+    pub fn from_probe_name(name: &str) -> Self {
+        match name {
+            "subrip" | "srt" => Self::SubRip,
+            "ass" | "ssa" => Self::Ass,
+            "mov_text" => Self::MovText,
+            "hdmv_pgs_subtitle" => Self::Pgs,
+            "dvd_subtitle" => Self::VobSub,
+            _ => Self::Other,
+        }
+    }
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
@@ -490,7 +507,7 @@ pub struct MovieOwner {
 
 #[cfg(test)]
 mod tests {
-    use super::{DolbyVisionProfile, HdrFormat, Language, Rule, Verdict};
+    use super::{DolbyVisionProfile, HdrFormat, Language, Rule, SubtitleCodec, Verdict};
 
     #[test]
     fn portuguese_variants_are_distinct() {
@@ -512,6 +529,26 @@ mod tests {
         );
     }
 
+    #[test]
+    fn probe_names_round_trip_through_the_codec() {
+        for codec in [
+            SubtitleCodec::SubRip,
+            SubtitleCodec::Ass,
+            SubtitleCodec::MovText,
+            SubtitleCodec::Pgs,
+            SubtitleCodec::VobSub,
+        ] {
+            assert_eq!(SubtitleCodec::from_probe_name(&codec.to_string()), codec);
+        }
+        assert_eq!(SubtitleCodec::from_probe_name("srt"), SubtitleCodec::SubRip);
+        assert_eq!(SubtitleCodec::from_probe_name("ssa"), SubtitleCodec::Ass);
+        assert_eq!(
+            SubtitleCodec::from_probe_name("dvb_subtitle"),
+            SubtitleCodec::Other
+        );
+        assert!(!SubtitleCodec::from_probe_name("other").is_text());
+    }
+
     #[test]
     fn non_eligible_verdicts_name_the_rule() {
         let rule = Rule::DolbyVisionProfile(DolbyVisionProfile {