Add Anchor: an invalid subpath is an error, not the last subpath

path.appendAnchor read `subpath` with as_u64, so -1 or "0" counted as omitted
and the point went onto the last subpath. Only an omitted (or null) subpath
means the last one now; any other value must be a valid index.

Found by CodeRabbit on #1657.

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

crates/engine/src/cmd/object.rs

@@ -3201,7 +3201,11 @@ fn path_create(s: &mut Session, p: &Value) -> Result<Value> {
 fn path_append(s: &mut Session, p: &Value) -> Result<Value> {
     let id = super::id_param(p, "id").ok_or_else(|| bad("path.appendAnchor", "missing id"))?;
     let a = p.get("anchor").and_then(anchor_from).ok_or_else(|| bad("path.appendAnchor", "missing anchor"))?;
-    let subpath = p.get("subpath").and_then(Value::as_u64);
+    // An omitted subpath is the last one; one given must name a subpath (never falls back).
+    let subpath = match p.get("subpath") {
+        None | Some(Value::Null) => None,
+        Some(v) => Some(v.as_u64().ok_or_else(|| bad("path.appendAnchor", "`subpath` must be a subpath index"))?),
+    };
     let at_start = match p.get("at") {
         None => false,
         Some(v) if v.as_str() == Some("end") => false,

crates/engine/src/tests.rs

@@ -1705,7 +1705,10 @@ fn selected_points_follow_their_path() {
     let (_, si, ai) = st.selection.anchors[0];
     let it = st.doc.item(id(l)).unwrap();
     assert_eq!((it.xf * it.path.subpaths[si].anchors[ai].p).round(), designcraft_geom::Point::new(200.0, 100.0));
-    assert!(s.execute("path.appendAnchor", &json!({"id": l, "anchor": {"p": [0, 0]}, "subpath": 3})).is_err(), "no such subpath");
+    for bad in [json!(3), json!(-1), json!("0")] {
+        assert!(s.execute("path.appendAnchor", &json!({"id": l, "anchor": {"p": [0, 0]}, "subpath": bad})).is_err(), "subpath {bad}");
+    }
+    assert_eq!(s.doc().unwrap().doc.item(id(l)).unwrap().path.subpaths[0].anchors.len(), 4, "nothing appended");
 }
 
 #[test]