feat(insights): populate the trailing sparkline series on derived metrics

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.

src/lib/openapi/routes.ts

@@ -1388,7 +1388,7 @@ const derivedMetricResponse = z
       .record(z.string(), z.unknown())
       .nullable()
       .describe(
-        "Metric-specific value object when status is 'ok' (e.g. { type, center, low, high, spread, sampleDays, k } for VITALS_BASELINE); null when 'insufficient'.",
+        "Metric-specific value object when status is 'ok' (e.g. { type, center, low, high, spread, sampleDays, k, series } for VITALS_BASELINE, where `series` is the trailing per-day mean values for the inline sparkline); null when 'insufficient'.",
       ),
     coverage: derivedCoverage,
     confidence: derivedConfidence