Measure overlap without double counting

Implement span_coverage(root, children) where each interval is (start,end) in integer milliseconds. Return {"root_ms": duration, "covered_ms": union_length, "uncovered_ms": remainder}. Children may overlap or be empty, and zero-length intervals are allowed. Reject reversed intervals and children outside the root. Compute the union rather than adding durations. The child union is coverage of this local timeline, not a complete distributed critical-path algorithm.