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,
 }