```ts
/**
 * Scans a string for bracket balance over (), [], {} pairs.
 * Returns -1 if balanced, otherwise the index of the first offending character.
 */
export function firstUnbalanced(input: string): number {
  const stack: { char: string; index: number }[] = [];
  const openingBrackets = new Set(['(', '[', '{']);
  const closingBrackets = new Set([')', ']', '}']);
  const bracketPairs: Record<string, string> = {
    ')': '(',
    ']': '[',
    '}': '{',
  };

  for (let i = 0; i < input.length; i++) {
    const char = input[i];

    if (openingBrackets.has(char)) {
      stack.push({ char, index: i });
    } else if (closingBrackets.has(char)) {
      if (stack.length === 0 || stack[stack.length - 1].char !== bracketPairs[char]) {
        return i;
      }
      stack.pop();
    }
  }

  if (stack.length > 0) {
    return stack[0].index;
  }

  return -1;
}
```