Name the failing step in batch command errors

Fixes #1331

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

crates/engine/src/batch_step_ref_tests.rs

@@ -110,3 +110,26 @@ fn a_nested_batch_refers_to_its_own_steps() {
     let rect = s.doc().unwrap().node(id(&inner[1])).unwrap().clone();
     assert_eq!(rect.parent, Some(id(&inner[0])), "in the nested batch's own first frame, not the outer one");
 }
+
+#[test]
+fn a_failing_command_names_its_step_and_rolls_back() {
+    let mut s = Session::new();
+    let frame = id(&s.execute("node.create", &json!({"type": "frame", "width": 50, "height": 50})).unwrap());
+    let text = id(&s.execute("node.create", &json!({"type": "text", "text": "orig"})).unwrap());
+    let undo = s.ds().unwrap().history.undo.len();
+    let name = s.doc().unwrap().node(s.page().unwrap()).unwrap().name.clone();
+    let r = s.execute(
+        "command.batch",
+        &json!({"commands": [
+            {"command": "text.set", "params": {"ids": [text.0], "text": "novo"}},
+            {"command": "text.set", "params": {"ids": [frame.0], "text": "erro"}},
+            {"command": "page.rename", "params": {"name": "Depois do erro"}}
+        ]}),
+    );
+    let msg = r.unwrap_err().to_string();
+    assert!(msg.contains("step 1: invalid parameters for"), "{msg}");
+    assert!(msg.contains("is not text"), "{msg}");
+    assert!(matches!(&s.doc().unwrap().node(text).unwrap().kind, figcraft_doc::NodeKind::Text(t) if t.characters == "orig"));
+    assert_eq!(s.doc().unwrap().node(s.page().unwrap()).unwrap().name, name);
+    assert_eq!(s.ds().unwrap().history.undo.len(), undo);
+}

crates/engine/src/lib.rs

@@ -1280,7 +1280,15 @@ impl Session {
     /// resolve `$step` the same way, inside its own [`Session::grouped`].
     pub fn batch_step(&mut self, results: &[Value], step: usize, cmd: &str, params: &Value) -> Result<Value> {
         let p = if cmd == "command.batch" { params.clone() } else { resolve_step_refs(params, results, step, 0)? };
-        self.execute(cmd, &p)
+        self.execute(cmd, &p).map_err(|e| match e {
+            // Say which step failed; a nested batch's error already does.
+            EngineError::UnknownCommand(_) | EngineError::BadParams { .. } | EngineError::NoNode(_) | EngineError::Other(_)
+                if !e.to_string().starts_with("step ") =>
+            {
+                EngineError::Other(format!("step {step}: {e}"))
+            }
+            e => e,
+        })
     }
 
     /// Run `f` as one undo step called `label`: the commands it executes push no steps of their