LRC-LIB-SMARTCOLL: album_reaches uses a hash set; loop test describes the guard

From review: the walk through smart-album references kept its seen-set
in a Vec, which is quadratic over long chains. The loop test's comment
now says what a saved loop does: inside it, the album being evaluated
counts as holding no photos.

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

crates/catalog/src/lib.rs

@@ -335,13 +335,12 @@ impl Catalog {
     /// rules of `from` would then change when those of `to` do). Loops in saved rules end the
     /// search, they don't repeat it.
     pub fn album_reaches(&self, from: AlbumId, to: AlbumId) -> bool {
-        let mut seen: Vec<AlbumId> = Vec::new();
+        let mut seen: std::collections::HashSet<AlbumId> = std::collections::HashSet::new();
         let mut next = vec![from];
         while let Some(a) = next.pop() {
-            if seen.contains(&a) {
+            if !seen.insert(a) {
                 continue;
             }
-            seen.push(a);
             let tested = self
                 .albums
                 .get(&a)

crates/catalog/src/tests.rs

@@ -522,7 +522,8 @@ fn a_smart_album_can_exclude_another() {
 
 /// An album can't include itself, directly or through other smart albums: the check refuses the
 /// loop for the album being edited, and a loop saved anyway (an older version, a hand-edited file)
-/// neither recurses forever nor crashes; it is reported and its rule matches nothing.
+/// neither recurses forever nor crashes: inside it, the album being evaluated counts as holding no
+/// photos, and every album in it is reported.
 #[test]
 fn smart_album_loops_are_refused_and_survived() {
     let mut c = Catalog::new();