fox 2026-05-21: account wattage for input and output separately. Prefill
(process all prompt tokens, parallel/compute-bound) and decode (generate
output, autoregressive/bandwidth-bound) are different GPU ops with
different J/token — a single per-token number can't represent both.
Slope calibration (no sub-request power alignment): sweep prompt length
at tiny max_tokens -> prefill J/input-tok (fixed overhead cancels in the
slope); fix a tiny prompt and sweep forced output length (ignore_eos) ->
decode J/output-tok. Prefill kept COLD (unique filler so cached_tokens=0).
Reuses watt_bench probes. Bad points (context overflow) skip, not abort.
Measured qwen-nothink/4090 @$0.33/kWh: prefill 0.175 J/tok
($0.016/M-input-tok), decode 6.16 J/tok ($0.564/M-output-tok) — decode
35x dearer per token. Predicts measured substrate J/q within ~5%. 14
tests (+ slope). Validated live.
