gen_queue: add in_flight_count tracking for all work item types (not
just AnalyzeBody). Rewrite saturation test with AnalyzeBody items.
Fix priority_ordering test thread count to match new gate.
rng: collapse to single AtlasRng::new(seed) constructor — callers
own their seed transform.
import_province_boundaries: fix "savepoint" comment to "transaction".
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>