```typescript

/** The words grouped by their length, in first-seen order. */
export function groupByLength(words: readonly string[]): Map<number, readonly string[]> {
  const groups = new Map();
  for (const word of words) {
    const bucket = groups.get(word.length);
    if (bucket === undefined) {
      groups.set(word.length, [word]);
    } else {
      bucket.push(word);
    }
  }

  // Return the Map with keys and values as Read-only Arrays
  return new Map([...groups.entries()]);
}
```