feat(insights): TRAJECTORY forecasting derived metric with a widening band

Add a short-horizon OLS projection over the DAY-native per-day mean
series, extending the existing trendSlope. The fit, residual standard
error and prediction interval are computed at DAY granularity only — the
rollup invariant forbids composing a slope/sd from WEEK/MONTH buckets.

The projection is gated on fit quality (R squared), in-window history and
staleness: below any floor the engine returns insufficient rather than a
weak line, so noise is never extrapolated. The prediction-interval band
widens with the horizon distance from the data centre, so the fan
visibly communicates uncertainty.

Register TRAJECTORY in the derived registry and dispatch (additive,
iOS-forward-compatible; the capabilities endpoint and drift-guard pick it
up automatically). Surface it on the weight detail page as a forecast
band beneath the chart, labelled a projection with the cited method and
a conditional, range-only headline — never a prediction or a dated
event. i18n across all six locales.

src/app/insights/weight/page.tsx

@@ -17,6 +17,7 @@ import { MeasurementDiversityNudge } from "@/components/insights/measurement-div
 import { MetricRangeControls } from "@/components/insights/metric-range-controls";
 import { MetricTargetSummary } from "@/components/insights/metric-target-summary";
 import { SubPageShell } from "@/components/insights/sub-page-shell";
+import { TrajectoryForecastCard } from "@/components/insights/derived/trajectory-forecast-card";
 import { buildWeightBandsFromHeight } from "@/lib/analytics/value-bands";
 
 /**
@@ -103,6 +104,13 @@ export default function InsightsGewichtPage() {
 
       <MetricTargetSummary slug="weight" />
 
+      <TrajectoryForecastCard
+        type="WEIGHT"
+        unit="kg"
+        color="#bd93f9"
+        enabled={!isEmpty}
+      />
+
       <InsightStatusCard
         title={t("insights.assessmentTitle")}
         icon={<Scale className="h-5 w-5" />}