Gradients: paint.setStroke refuses a dithered gradient when "swatch" is null

Review follow-up (CodeRabbit): the refusal skipped any request carrying a
`swatch` key, so `"swatch": null` with a dithered gradient got through.
Only a named swatch (a gradient swatch keeps its own dither) is exempt now.
The dither test sends `"swatch": null`; it fails without the change.

crates/engine/src/cmd/paint.rs

@@ -403,8 +403,9 @@ fn set_paint(s: &mut Session, p: &Value, fill: bool) -> Result<Value> {
     }
     check_keep_model(p, cmd)?;
     let paint = paint_from(s, p)?.ok_or_else(|| bad(cmd, "give color, none, swatch or gradient"))?;
-    // As `paint.editGradient`: a `dither` given applies to gradient fills only.
-    if !fill && matches!(&paint, Paint::Gradient(g) if g.gradient.dither) && p.get("swatch").is_none() {
+    // As `paint.editGradient`: a `dither` given applies to gradient fills only. A gradient swatch
+    // (named) keeps its own dither; `"swatch": null` names none.
+    if !fill && matches!(&paint, Paint::Gradient(g) if g.gradient.dither) && p.get("swatch").and_then(Value::as_str).is_none() {
         return Err(bad(cmd, "`dither` applies to gradient fills (paint.setFill)"));
     }
     apply_paint(s, p, paint, fill)

crates/engine/src/tests_gradient.rs

@@ -441,6 +441,7 @@ fn dither_is_a_fill_gradient_option_that_saves_and_loads() {
     let before = node(&s, id).appearance.clone();
     assert!(s.execute("paint.editGradient", &json!({"dither": true, "stroke": true})).is_err());
     assert!(s.execute("paint.setStroke", &json!({"gradient": {"dither": true}})).is_err());
+    assert!(s.execute("paint.setStroke", &json!({"swatch": null, "gradient": {"dither": true}})).is_err(), "a null swatch names none");
     assert!(s.execute("paint.editGradient", &json!({"dither": "yes"})).is_err());
     assert!(s.execute("paint.setFill", &json!({"gradient": {"dither": 1}})).is_err());
     assert_eq!(node(&s, id).appearance, before);