Stress: Segment Sum Oracle

**Bài toán.** Two implementations of max segment sum are compiled below: a candidate and an O(n²) oracle. Run a differential harness of 3000 random trials (seeds 12345..., lengths 1–9, four value distributions) and print OK 3000 if every trial agrees, or MISMATCH trial <t> at the first disagreement. **The candidate below is deliberately broken in one small way — your harness must expose it.** Fix it so the full loop prints OK.