gather
Collect per-experiment observation arrays for responseName, restricted to the experiments that record it. Iteration order matches selectedExperiments — i.e. source-order then natural order within each source — which is the canonical left→right ordering for cross-experiment box plots and CI plots. Returns an empty map when no experiment records the response.
This is the canonical input to the host-side comparison report renderer (e.g. ComparisonReportRenderer.renderBoxPlot / renderMca / renderCiPlot).