NAME THE TWO SIGNALS with standard_peak_ppm and analyte_peak_ppm. Without
them the first two integration regions are used, which is a guess - the
response then carries selection.method="positional" and a warning.

Optional analyte_signals, e.g. [{"ppm": 3.80, "protons": 3, "label": "OCH3"},
{"ppm": 1.20, "protons": 6}], quantifies the analyte on each listed signal and
returns `multi_signal`: per-signal concentrations, mean, SD, RSD, and the
signals that disagree with the others by more than the integration
uncertainty allows - the usual sign that a signal overlaps an impurity. The
headline result stays the one from analyte_peak_ppm.

Until 2026-07-31 this wrapper passed `reference_peak_ppm` and the core never
read it, while the core silently took integrals[0] and integrals[1]. The
ratio was effectively arbitrary and the certificate looked the same either
way. `reference_peak_ppm` is still accepted and is now honoured as the
standard's shift, so existing callers stop being ignored.

The response includes a `selection` block naming the regions that were
integrated, so the number can be checked against the spectrum.