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>

server/src/atlas/rng.rs

@@ -3,10 +3,7 @@
 //! Shared by all atlas generation modules that need seeded randomness.
 //! Uses Knuth's LCG parameters — integer-only arithmetic, no f32, D-010 compliant.
 //!
-//! Two construction options:
-//! - [`AtlasRng::new`] — adds 1 to seed (district_mix convention).
-//! - [`AtlasRng::new_mixed`] — mixes seed with a Fibonacci hash constant to
-//!   avoid degenerate state at 0 (skeleton_gen convention).
+//! Callers are responsible for any seed pre-mixing before calling `AtlasRng::new`.
 
 /// Seeded linear congruential generator (D-010).
 pub struct AtlasRng {
@@ -14,18 +11,12 @@ pub struct AtlasRng {
 }
 
 impl AtlasRng {
-    /// Seed by adding 1 — matches the district_mix convention.
+    /// Create a new RNG from a pre-mixed seed.
+    ///
+    /// Callers must ensure the seed is non-degenerate (avoid passing 0 directly
+    /// if the seed could realistically be 0 — add a constant before calling).
     pub fn new(seed: u64) -> Self {
-        Self {
-            state: seed.wrapping_add(1),
-        }
-    }
-
-    /// Seed with Fibonacci hash mix — avoids degenerate state at seed=0.
-    pub fn new_mixed(seed: u64) -> Self {
-        Self {
-            state: seed.wrapping_add(0x9e37_79b9_7f4a_7c15),
-        }
+        Self { state: seed }
     }
 
     fn next_u64(&mut self) -> u64 {
@@ -63,12 +54,4 @@ mod tests {
         let vals_b: Vec<u32> = (0..10).map(|_| b.next_u32()).collect();
         assert_ne!(vals_a, vals_b);
     }
-
-    #[test]
-    fn mixed_seed_avoids_zero_state() {
-        let mut r = AtlasRng::new_mixed(0);
-        // Should not produce a constant zero sequence.
-        let vals: Vec<u32> = (0..5).map(|_| r.next_u32()).collect();
-        assert!(vals.iter().any(|&v| v != 0));
-    }
 }