The Derived<T> tile (SparklineDeltaTile) renders a trailing series, but the
engines never filled it, so the inline sparkline was always empty. Each engine
now carries a capped trailing series sourced from the rows it already reads:
- baseline-band metrics (incl. wrist-temperature / stair ascent+descent) ship
the per-day mean series the band is built from;
- HRV balance reuses the day-mean read it already does for the recent average;
- the wellness scores (recovery / stress / strain) ship their window scores;
- the six-minute-walk band ships its reading series;
- BMI reads a bounded recent-weight set and derives a BMI series at the fixed
profile height.
All series cap at the last 30 points. The vitals dashboard wires the series
into the baseline, HRV and BMI tiles; the tile already renders >= 2 points and
collapses the sparkline row otherwise.