Engine: Interpret Footage loop coverage for sequences (#1068)

Looped *video* footage had a regression (#1045); a grouped PNG sequence
going through the same path — interpret a fresh placeholder to Loop 15,
drag it onto the timeline — did not. The behaviour is already correct on
main (addItem sizes a sequence layer from looped_duration, the pool's
frame_index wraps per cycle), so this is coverage, not a change:
interpret_footage_loop_extends_sequence_layers asserts a 15-loop of a
0.4 s sequence lands a 6 s layer.

cargo test -p effectcraft-engine --lib interpret_footage: 4 passed.

crates/engine/src/tests_menu_cmds.rs

@@ -1556,6 +1556,31 @@ fn interpret_footage_loop_extends_video_layers() {
     assert_eq!(s.project.item(effectcraft_project::ItemId(p)).unwrap().duration(), Some(Tick::from_seconds_f64(6.0)));
 }
 
+/// #1068: the same for an interpreted PNG sequence — the loop count survives interpretation and
+/// a freshly dragged layer plays every cycle.
+#[test]
+fn interpret_footage_loop_extends_sequence_layers() {
+    let mut s = comp();
+    let p = s.execute("file.importPlaceholder", json!({"name": "Seq", "width": 320, "height": 240, "duration": 0.4})).unwrap()["item"].as_u64().unwrap();
+    let id = effectcraft_project::ItemId(p);
+    // Make it a sequence like the importer's grouped PNGs.
+    s.edit("Seq", None, |proj, _| {
+        if let Some(ItemKind::Footage(f)) = proj.item_mut(id).map(|i| &mut i.kind) {
+            f.kind = effectcraft_project::FootageKind::Sequence;
+            f.sequence = (0..12).map(|n| format!("/tmp/seq_{n:04}.png")).collect();
+            f.sync_sequence_duration();
+        }
+        Ok(())
+    })
+    .unwrap();
+    s.state.project_selection = vec![id];
+    s.execute("file.interpretFootage", json!({"loop": 15.0})).unwrap();
+    let l = s.execute("layer.addItem", json!({"item": p})).unwrap()["layer"].as_u64().unwrap();
+    let layer = s.active_comp().unwrap().layer(effectcraft_project::LayerId(l)).unwrap();
+    let dur = (layer.out_point - layer.in_point).seconds();
+    assert!((dur - 6.0).abs() < 0.05, "15 loops of a 0.4 s sequence: {dur}");
+}
+
 #[test]
 fn percentage_guides_follow_the_comp_size() {
     let mut s = comp();