fix(level): pass timestamp on level samples for level-rate fallback

MeasurementRouter.onLevelMeasurement was writing level samples via
.value(value).unit(context.unit), which dropped the timestamp. The
level-rate fallback in FlowAggregator derives netFlow from dlevel/dt,
so without a timestamp on each sample it had nothing to differentiate.

Switch to the positional .value(value, timestamp, unit) form so the
fallback works. Add a basic test that drives two level samples 2 s
apart and asserts the aggregator produces direction=filling with a
finite dlevel/dt-derived netFlow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/measurement/measurementRouter.js

@@ -34,7 +34,7 @@ class MeasurementRouter {
 
   onLevelMeasurement(position, value, context = {}) {
     this.measurements.type('level').variant('measured').position(position)
-      .value(value).unit(context.unit);
+      .value(value, context.timestamp, context.unit);
 
     const series = this.measurements.type('level').variant('measured').position(position);
     const levelMeters = series.getCurrentValue('m');