is_balanced parser — handle all bracket types

def is_balanced(s: str) -> bool:
    """Return True iff every bracket in s is matched and properly nested."""
    pairs = {")": "(", "]": "[", "}": "{"}
    stack: list[str] = []
    for ch in s:
        if ch in "([{":
            stack.append(ch)
        elif ch in pairs:
            if not stack or stack.pop() != pairs[ch]:
                return False
    return not stack