M3.21: active-row edits of the other kind stay on the painted leaves, item transparency is one undo step

- A fill row no longer turns stroke edits (stroke.set, setAdvanced, paint.setStroke, gradients) into
  a new stroke on a selected group's own stack: an active row only stands for edits of its kind.
- Removing, duplicating or moving items of other objects (`ids`) no longer re-points the active row.
- transparency.set with an item touches the item only for opacity/blend (isolate or knockout alone
  is one undo step) and rejects unknown blend modes.

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

crates/engine/src/cmd/stroke.rs

@@ -35,7 +35,7 @@ fn stroke_set(s: &mut Session, p: &Value) -> Result<Value> {
     if let Some(w) = p.get("weight").and_then(Value::as_f64) {
         s.paint.stroke_width = w.max(0.0);
     }
-    let item = item_target(s, p, C)?;
+    let item = item_target(s, p, C)?.of_kind(s, false);
     let ids = item.targets(s, p)?;
     let arrow = |k: &str| -> Result<Option<Option<Arrowhead>>> {
         match p.get(k) {
@@ -139,7 +139,7 @@ fn stroke_advanced(s: &mut Session, p: &Value) -> Result<Value> {
     if scale.is_none() && flip.is_none() && !swap && brush.is_none() {
         return Err(bad(C, "nothing to change"));
     }
-    let item = item_target(s, p, C)?;
+    let item = item_target(s, p, C)?.of_kind(s, false);
     let ids = item.targets(s, p)?;
     edit_items(s, &ids, item, C, "Stroke", false, |n, index| {
         let Some(st) = n.appearance.stroke_at_mut(index) else { return Ok(()) };