Function boxplotSelectors

Generates per-box BoxSelector entries for a Highcharts boxplot series.

MAIDR's BoxTrace expects one BoxSelector per box (length must equal the number of data points) rather than a single CSS selector string. Each entry names the sub-elements (iq, q2, min, max, optional outliers) so Svg.selectElement / Svg.selectAllElements can locate them individually.

Highcharts renders each box as:

<g class="highcharts-point">
  <path class="highcharts-boxplot-stem"   .../>
  <path class="highcharts-boxplot-whisker" d="M ... cap1 ... M ... cap2" />
  <path class="highcharts-boxplot-box"     .../>  <!-- IQR body -->
  <path class="highcharts-boxplot-median"  .../>
</g>
Copy

Caller (the adapter) is responsible for stamping each g.highcharts-point with data-maidr-box-index="N" AND splitting the whisker <path> into data-maidr-box-part="upper-whisker" | "lower-whisker" siblings so these selectors are stable and per-box. See stampBoxIndices / splitWhiskerPath in adapter.ts.

Trade-off: outliers are returned as empty arrays. Highcharts treats outliers as a sibling scatter series, not children of the boxplot group; supporting them would require a separate adapter pass.