Merge K Sorted Runs

Merge a list of sorted integer runs into one ascending list. Preserve duplicate values. Runs can be empty; there may be zero runs. Each run is already nondecreasing. Use a heap with at most one pending value per nonempty run, rather than flattening and sorting all values. Return values only, so equal values are indistinguishable. Examples (JSON; null means Python None): Input: {"runs": [[1, 4], [1, 3, 5], [2, 6]]} Output: [1, 1, 2, 3, 4, 5, 6] Input: {"runs": [[], [-2, 0], []]} Output: [-2, 0]

def merge_runs(runs: list[list[int]]) -> list[int]:
    """Merge sorted runs using a bounded frontier."""
    pass