Ensemble percentiles (subset)

### What problem does your feature request solve?

Percentiles based on the subset of ensembles.

### Describe the solution you'd like

Recipe.

### Describe alternatives you've considered

The issue does not name an implementation file, test, or entry point. Start by locating the existing ensemble percentile recipe and how ensemble subsets are represented, then clarify the intended subset-selection behavior with maintainers. Done should mean a recipe can calculate percentiles from a selected ensemble subset with coverage for the expected inputs.

Written by the indexing model from the issue text.