D137 - GPU diffs are keyed by input, not lane

Status: decided Date: 2026-09-26

obscene-tool gpudiff keys each lane by (kernel, inputs), prints both corpora's gpudev first and warns when they are the same device, and reports a lane present on only one side as a divergence.

Why: the operands are a lane's identity and what a person fixing an emulator needs. Diffing a device against itself measures nondeterminism, not a gap. A one-sided lane means the runs did not cover the same ground.

Rejected: keying by lane index - breaks on any change to lane order.