Pipeline timings say build_subsets.py takes longer than the census; it takes about 6 minutes to the census's 8

[realmarcin]

From the round-2 review of #216.

Where: .claude/skills/update-xmech-page/SKILL.md step 4 on docs/skill-snapshot-cleanup ("longer than the census"), and _fleet/README.md:126 on main ("The census takes about eight minutes and build_subsets.py longer").

The ten-Mech stage logs at the #120 pins show build_subsets.py finishing faster than the census:

run	census	build_subsets.py
census87.log	501 s
subsets87.log		362 s
subsets_labels.log		347 s
subsets149.log		about 481 s, alongside another scan

Why it matters: an agent told subsets take longer than eight minutes won't notice a stalled run.

Fix: census about 8 minutes, build_subsets.py about 6, in both places.