{
  "row": [
    1,
    1,
    2
  ],
  "R_first": [
    3,
    -1,
    2
  ],
  "A_first": [
    1,
    2,
    3
  ],
  "common_endpoint": [
    3,
    0,
    3
  ],
  "status": "pure algebraic diagnostic; not a chronological row",
  "interpretation": "A_first remains nonnegative through both steps; R_first leaves the domain despite the same valid endpoint"
}