Fix add overflow panic in table.columnWidth for a huge column index (#922)

Found while testing crates/engine/src/cmd/table.rs:540 (column_width)

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/table.rs

@@ -537,7 +537,7 @@ fn column_width(s: &mut Session, p: &Value) -> Result<Value> {
     with_table(s, p, "table.columnWidth", move |t| {
         // Keeping the table's width, the column to the right gives or takes the difference (dragging
         // an inner border). The last column has none: the table gets wider or narrower.
-        let pair = t.cols.get(c).zip(t.cols.get(c + 1)).map(|(a, b)| a.max(0.0) + b.max(0.0)).filter(|x| x.is_finite());
+        let pair = t.cols.get(c).zip(t.cols.get(c.saturating_add(1))).map(|(a, b)| a.max(0.0) + b.max(0.0)).filter(|x| x.is_finite());
         match pair {
             Some(pair) if keep_total => {
                 let min = MIN_COLUMN_WIDTH.min(pair / 2.0);
@@ -545,7 +545,7 @@ fn column_width(s: &mut Session, p: &Value) -> Result<Value> {
                 if let Some(x) = t.cols.get_mut(c) {
                     *x = w;
                 }
-                if let Some(x) = t.cols.get_mut(c + 1) {
+                if let Some(x) = t.cols.get_mut(c.saturating_add(1)) {
                     *x = pair - w;
                 }
             }

crates/engine/src/tests.rs

@@ -3612,3 +3612,14 @@ fn table_insert_below_or_right_of_a_huge_index_appends_at_the_end() {
     assert_eq!(t.n_cols(), 3);
     assert_eq!(t.cell(0, 1).unwrap().text.text(), "b", "the original columns keep their place");
 }
+
+#[test]
+fn table_column_width_of_a_huge_column_is_ignored() {
+    let mut s = session();
+    let id = table_at(&mut s, &[100.0, 100.0], json!([["a", "b"], ["c", "d"]]));
+    for keep_total in [true, false] {
+        let r = s.execute("table.columnWidth", &json!({"id": id.0, "col": u64::MAX, "width": 50.0, "keepTotal": keep_total}));
+        assert!(r.is_ok(), "columnWidth must ignore a huge col (keepTotal {keep_total}), got {r:?}");
+        assert_eq!(table_of(&s, id).cols, vec![100.0, 100.0], "no column changes");
+    }
+}