test(history): end-to-end ConvergenceOptions propagation tests

Two integration tests on a 4-team ranked event:
- max_iter=1 set on HistoryBuilder produces measurably different
  posteriors than default, proving the inner loop honors the
  propagated max_iter
- alpha=0.5 with extra iterations reaches the same fixed point as
  alpha=1.0, proving damping doesn't break correctness on the History
  path

Also updates the alpha doc comment to clarify it applies only to the
within-game EP loop, not the outer cross-history sweep.

src/convergence.rs

@@ -9,9 +9,14 @@ pub struct ConvergenceOptions {
     pub max_iter: usize,
     pub epsilon: f64,
     /// EP damping factor in natural-parameter space: each per-factor
-    /// update writes `α·new + (1−α)·old`. `1.0` is undamped (default);
-    /// `< 1.0` stabilises oscillating fixed-point loops at the cost of
-    /// more iterations. Must be in `(0.0, 1.0]`.
+    /// update inside a single game writes `α·new + (1−α)·old`. `1.0` is
+    /// undamped (default); `< 1.0` stabilises oscillating fixed-point
+    /// loops at the cost of more iterations. Must be in `(0.0, 1.0]`.
+    ///
+    /// Applies only to the within-game EP loop (`run_chain`). The outer
+    /// `History::converge` cross-history sweep is undamped regardless of
+    /// this value — cross-slice damping is a different concept and not
+    /// in scope.
     pub alpha: f64,
 }
 

src/history.rs

@@ -1714,4 +1714,97 @@ mod tests {
     fn history_builder_rejects_zero_score_sigma() {
         let _ = History::builder().score_sigma(0.0).build();
     }
+
+    #[test]
+    fn history_propagates_convergence_to_inner_run_chain() {
+        use crate::ConvergenceOptions;
+
+        let events_for =
+            |h: &mut History<i64, ConstantDrift, crate::observer::NullObserver, &'static str>| {
+                h.event(0)
+                    .team(["a"])
+                    .team(["b"])
+                    .team(["c"])
+                    .team(["d"])
+                    .ranking([0u32, 1, 2, 3])
+                    .commit()
+                    .unwrap();
+            };
+
+        let mut h_capped: History<i64, _, _, &'static str> = History::builder()
+            .convergence(ConvergenceOptions {
+                max_iter: 1,
+                ..ConvergenceOptions::default()
+            })
+            .build();
+        events_for(&mut h_capped);
+        h_capped.converge().unwrap();
+
+        let mut h_full: History<i64, _, _, &'static str> = History::builder().build();
+        events_for(&mut h_full);
+        h_full.converge().unwrap();
+
+        let curves_capped = h_capped.learning_curves();
+        let curves_full = h_full.learning_curves();
+
+        let mut max_diff: f64 = 0.0;
+        for (key, capped_pts) in curves_capped.iter() {
+            let full_pts = curves_full.get(key).expect("agent missing in full");
+            for (capped, full) in capped_pts.iter().zip(full_pts.iter()) {
+                max_diff = max_diff.max((capped.1.mu() - full.1.mu()).abs());
+                max_diff = max_diff.max((capped.1.sigma() - full.1.sigma()).abs());
+            }
+        }
+        assert!(
+            max_diff > 1e-6,
+            "max_iter=1 inner loop should differ from default; max_diff={max_diff}"
+        );
+    }
+
+    #[test]
+    fn history_with_damping_reaches_same_fixed_point_as_undamped() {
+        use crate::ConvergenceOptions;
+
+        let events_for =
+            |h: &mut History<i64, ConstantDrift, crate::observer::NullObserver, &'static str>| {
+                h.event(0)
+                    .team(["a"])
+                    .team(["b"])
+                    .team(["c"])
+                    .team(["d"])
+                    .ranking([0u32, 1, 2, 3])
+                    .commit()
+                    .unwrap();
+            };
+
+        let mut h_undamped: History<i64, _, _, &'static str> = History::builder().build();
+        events_for(&mut h_undamped);
+        h_undamped.converge().unwrap();
+
+        let mut h_damped: History<i64, _, _, &'static str> = History::builder()
+            .convergence(ConvergenceOptions {
+                alpha: 0.5,
+                max_iter: 200,
+                ..ConvergenceOptions::default()
+            })
+            .build();
+        events_for(&mut h_damped);
+        h_damped.converge().unwrap();
+
+        let curves_u = h_undamped.learning_curves();
+        let curves_d = h_damped.learning_curves();
+
+        let mut max_diff: f64 = 0.0;
+        for (key, u_pts) in curves_u.iter() {
+            let d_pts = curves_d.get(key).expect("agent missing in damped");
+            for (u, d) in u_pts.iter().zip(d_pts.iter()) {
+                max_diff = max_diff.max((u.1.mu() - d.1.mu()).abs());
+                max_diff = max_diff.max((u.1.sigma() - d.1.sigma()).abs());
+            }
+        }
+        assert!(
+            max_diff < 1e-3,
+            "α=0.5 should reach the same fixed point as α=1.0; max_diff={max_diff}"
+        );
+    }
 }