Reject negative and fractional index in file.openRecent

Found while testing crates/engine/src/cmd/file/open.rs:33 (open_recent, `.nth(i.max(0.0) as usize)`)

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

crates/engine/src/cmd/file/open.rs

@@ -31,7 +31,10 @@ pub(super) fn open_recent(s: &mut Session, p: &Value) -> Result<Value> {
             Some(path)
         }
         (None, Some(i)) => {
-            Some(s.prefs.recent_files.iter().filter(|r| listed(r)).nth(i.max(0.0) as usize).cloned().ok_or_else(|| bad(CMD, "no such recent file"))?)
+            if !(i >= 0.0 && i.fract() == 0.0) {
+                return Err(bad(CMD, "no such recent file"));
+            }
+            Some(s.prefs.recent_files.iter().filter(|r| listed(r)).nth(i as usize).cloned().ok_or_else(|| bad(CMD, "no such recent file"))?)
         }
         _ => None,
     };

crates/engine/src/menu_gaps_tests.rs

@@ -84,3 +84,20 @@ fn outlined_layers_draw_as_outlines_and_the_rest_normally() {
     assert_eq!(outlined.pixel(20, 20).unwrap(), empty, "only its outline");
     assert_eq!(outlined.pixel(80, 20).unwrap(), plain.pixel(80, 20).unwrap(), "the other layer draws normally");
 }
+
+#[test]
+fn open_recent_rejects_negative_and_fractional_index() {
+    let dir = std::env::temp_dir().join(format!("figcraft-recent-bad-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let a = dir.join("a.figcraft").to_string_lossy().to_string();
+    let b = dir.join("b.figcraft").to_string_lossy().to_string();
+    let mut s = Session::new();
+    s.execute("file.save", &json!({"path": a})).unwrap();
+    s.execute("file.saveAs", &json!({"path": b})).unwrap();
+    let recent = s.prefs.recent_files.clone();
+    for i in [json!(-1), json!(1.5), json!(-0.5)] {
+        assert!(s.execute("file.openRecent", &json!({"index": i})).is_err(), "index {i} must be an error");
+    }
+    assert_eq!(s.prefs.recent_files, recent);
+    let _ = std::fs::remove_dir_all(&dir);
+}