Keep auto-advance when transition.timing gets an invalid after

Found while testing crates/engine/src/cmd/transition.rs:112-115 (timing)

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

crates/engine/src/cmd/transition.rs

@@ -111,7 +111,11 @@ fn timing(s: &mut Session, p: &Value) -> Result<Value> {
                 }
                 match p.get("after") {
                     Some(Value::Null) => t.advance_after_ms = None,
-                    Some(v) => t.advance_after_ms = v.as_u64().map(|x| x.min(86_400_000) as u32),
+                    Some(v) => {
+                        if let Some(x) = v.as_u64() {
+                            t.advance_after_ms = Some(x.min(86_400_000) as u32);
+                        }
+                    }
                     None => {}
                 }
             }

crates/engine/src/tests.rs

@@ -5406,3 +5406,17 @@ fn slide_hide_and_layout_reject_invalid_index() {
     }
     assert!(s.doc().unwrap().doc.slides.iter().all(|x| !x.hidden));
 }
+
+#[test]
+fn transition_timing_invalid_after_keeps_value() {
+    let mut s = session();
+    let after = |s: &Session| s.doc().unwrap().doc.slides[0].transition.as_ref().and_then(|t| t.advance_after_ms);
+    s.execute("transition.timing", &json!({"after": 5000})).unwrap();
+    assert_eq!(after(&s), Some(5000));
+    for bad in [json!(-5), json!(1.5), json!("abc")] {
+        let _ = s.execute("transition.timing", &json!({"after": bad}));
+        assert_eq!(after(&s), Some(5000), "after:{bad} must not clear the auto-advance");
+    }
+    s.execute("transition.timing", &json!({"after": null})).unwrap();
+    assert_eq!(after(&s), None);
+}