Refuse negative or fractional indexes in page.select and doc.switch

Found while testing crates/engine/src/cmd/page.rs:50

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

crates/engine/src/cmd/file.rs

@@ -219,7 +219,7 @@ pub fn specs() -> Vec<CommandSpec> {
             }
         ),
         spec!("doc.switch", "Switch document", "{index}", false, |s, p| {
-            let i = req_num("doc.switch", p, "index")? as usize;
+            let i = req_index("doc.switch", p, "index")?;
             if i >= s.docs.len() {
                 return Err(bad("doc.switch", "no such document"));
             }

crates/engine/src/cmd/page.rs

@@ -47,7 +47,7 @@ pub fn specs() -> Vec<CommandSpec> {
             let id = match id(p, "id") {
                 Some(i) => i,
                 None => {
-                    let k = req_num("page.select", p, "index")? as usize;
+                    let k = req_index("page.select", p, "index")?;
                     *doc.pages.get(k).ok_or_else(|| bad("page.select", "no such page"))?
                 }
             };

crates/engine/src/panels_behaviour_tests.rs

@@ -337,6 +337,23 @@ fn pages_by_the_documented_rules() {
     assert_ne!(s.page().unwrap(), div, "deleting the open page opens a page, never a divider");
 }
 
+#[test]
+fn page_select_and_doc_switch_refuse_negative_or_fractional_indexes() {
+    let mut s = Session::new();
+    let p2 = id(&ex(&mut s, "page.add", json!({})));
+    for bad_index in [json!(-1), json!(0.5)] {
+        assert!(s.execute("page.select", &json!({"index": bad_index})).is_err(), "page.select index {bad_index} must fail");
+        assert_eq!(s.page().unwrap(), p2, "page.select index {bad_index} leaves the open page alone");
+    }
+    ex(&mut s, "file.new", json!({}));
+    let active = s.active;
+    assert_eq!(active, 1);
+    for bad_index in [json!(-1), json!(0.5)] {
+        assert!(s.execute("doc.switch", &json!({"index": bad_index})).is_err(), "doc.switch index {bad_index} must fail");
+        assert_eq!(s.active, active, "doc.switch index {bad_index} leaves the active tab alone");
+    }
+}
+
 #[test]
 fn assets_panel_lists_files_and_drops_by_the_documented_rules() {
     let mut s = Session::new();