fix(simulation): address PR #142 review round 2

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>

tooling/planet-gen/import_province_boundaries.py

@@ -444,7 +444,7 @@ def import_body_provinces(
         print(f"    {body_id}: {len(basins)} basins — {', '.join(areas)}")
 
     if not dry_run:
-        with conn:  # per-body savepoint: rolls back this body on exception, keeps prior commits
+        with conn:  # per-body transaction (BEGIN/COMMIT): rolls back this body on exception, keeps prior commits
             conn.execute(
                 "DELETE FROM atlas_province_boundaries WHERE body_id = ?",
                 (body_id,),