Rebuild: an image without words around it is keyed as a stem

So a cover 'x.jpg' joins 'x.part01.rar' in a stem-named post; an image
set like 'IMG_008' keys as the stem 'IMG'. Both implementations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/edge-bench-rs/src/names.rs

@@ -534,7 +534,7 @@ pub fn release_identity(poster: &str, subject: &str, name: &str) -> Release {
     }
     if image {
         let name = if prefix.is_empty() { stem.clone() } else { prefix.clone() };
-        return Release { key: hash(&["images", poster, &prefix]), name, source: "stem", file_total };
+        return Release { key: hash(&["stem", poster, &name]), name, source: "stem", file_total };
     }
     Release { key: hash(&["stem", poster, &stem]), name: stem, source: "stem", file_total }
 }
@@ -947,6 +947,8 @@ mod tests {
         let par2 = id(r#"(Geisterjäger John Sinclair Hörspiele Vol. 2) [59/59] - "Hörspiele Vol. 2.par2" yEnc"#, "Hörspiele Vol. 2.par2");
         assert_eq!(t1.key, t2.key);
         assert_eq!(t1.key, par2.key);
+        // A cover without words around it joins its stem's archive.
+        assert_eq!(id(r#"[2/6] - "x.jpg" yEnc"#, "x.jpg").key, id(r#"[1/6] - "x.part01.rar" yEnc"#, "x.part01.rar").key);
     }
 
     #[test]