Measure error quantiles on a frozen synthetic benchmark

Measure error quantiles on a frozen synthetic benchmark

Generate a frozen set of synthetic populations from a recorded seed, run the mechanism a fixed number of times per population, and record the absolute error of every release. Deliverable: A CSV with population_id, true_count, released_count, and absolute_error, plus a quantile table at the 50th, 90th, and 99th percentiles.