[tamnd]

Part of tamnd/rudb#1387 (W4), the index gate of 17-milestones.md: building the key of a load of 10 million rows adds at most a tenth to the same load without it.

--mode index loads --records rows of (id BIGINT, v BIGINT) into a fresh table without a key and into one with id as its primary key, --runs times each, and drops both after each load. The two take turns, and which one goes first alternates by run, so neither always meets a file that just grew. The gate is judged on the medians. It runs on rudb, DuckDB and PostgreSQL, each loading from its own row generator.

On server3 with rudb and the W4 key work, 10 million rows over 5 runs: a median of 3.361 s without the key and 3.425 s with it, 1.9% added, a pass. The box was under a load of about 80 from other jobs, so single runs swing by tens of percent either way and only the medians mean much.

Tests: the median of an odd and an even count, and of nothing.