Fix flaky XML round-trip CI failure

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

src/io/xml.ts

@@ -417,22 +417,24 @@ export function readXml(text: string, options: ReadXmlOptions = {}): DataFrame {
   }
 
   // Build column arrays
-  const colData: Record<string, Scalar[]> = {};
-  for (const col of cols) {
-    colData[col] = rows.map((row) => {
-      const raw = row[col] ?? null;
-      if (raw === null || naSet.has(raw)) {
-        return null;
-      }
-      if (converters) {
-        const n = Number(raw);
-        if (!Number.isNaN(n) && raw.trim() !== "") {
-          return n;
+  const colData = Object.fromEntries(
+    cols.map((col) => [
+      col,
+      rows.map((row) => {
+        const raw = row[col] ?? null;
+        if (raw === null || naSet.has(raw)) {
+          return null;
         }
-      }
-      return raw;
-    });
-  }
+        if (converters) {
+          const n = Number(raw);
+          if (!Number.isNaN(n) && raw.trim() !== "") {
+            return n;
+          }
+        }
+        return raw;
+      }),
+    ]),
+  );
 
   // Determine index
   let idxCol: string | null = null;
@@ -445,10 +447,7 @@ export function readXml(text: string, options: ReadXmlOptions = {}): DataFrame {
   if (idxCol !== null && cols.includes(idxCol)) {
     const idxData = colData[idxCol] ?? [];
     const dataColNames = cols.filter((c) => c !== idxCol);
-    const dataColData: Record<string, Scalar[]> = {};
-    for (const c of dataColNames) {
-      dataColData[c] = colData[c] ?? [];
-    }
+    const dataColData = Object.fromEntries(dataColNames.map((c) => [c, colData[c] ?? []]));
     const idx = new Index(idxData.filter(isLabel));
     return DataFrame.fromColumns(dataColData, { index: idx });
   }

tests/io/xml.test.ts

@@ -311,6 +311,14 @@ describe("toXml / readXml round-trip", () => {
     expect(df2.col("id").toArray()).toEqual([1, 2]);
     expect(df2.col("name").toArray()).toEqual(["Alice", "Bob"]);
   });
+
+  test("round-trips __proto__ column names", () => {
+    const df = DataFrame.fromColumns(Object.fromEntries([["__proto__", ["Alice", "Bob"]]]));
+    const xml = toXml(df, { xmlDeclaration: false });
+    const df2 = readXml(xml, { converters: false });
+    expect(df2.shape).toEqual(df.shape);
+    expect(df2.col("__proto__").toArray()).toEqual(["Alice", "Bob"]);
+  });
 });
 
 // ─── property-based tests ─────────────────────────────────────────────────────
@@ -327,10 +335,9 @@ describe("readXml / toXml — property tests", () => {
         fc.integer({ min: 1, max: 5 }),
         (colNames, nRows) => {
           const uniqueCols = [...new Set(colNames)];
-          const colData: Record<string, string[]> = {};
-          for (const c of uniqueCols) {
-            colData[c] = Array.from({ length: nRows }, (_, i) => `v${i}`);
-          }
+          const colData = Object.fromEntries(
+            uniqueCols.map((c) => [c, Array.from({ length: nRows }, (_, i) => `v${i}`)]),
+          );
           const df = DataFrame.fromColumns(colData);
           const xml = toXml(df);
           const df2 = readXml(xml, { converters: false });