Scalars are the one part of the API keyed differently: by sensor, not
by stream. Nothing declares which measuring point a scalar belongs
to — the link is derived, and each row below renders that derivation,
hop by hop, as far as it reaches. Per-axis scalars (rms,
velocity_rms) carry a sensor_stream_id naming one
axis, so they resolve to one source and one H/V/A slot; sensor-level scalars
(temperature, battery, rpm) describe
the whole sensor and belong to the measuring point itself.
rpm matters more than it looks: outside 600–3600 RPM an
rpm scalar is required — without it, bearing analysis silently
returns nothing.