feat(convergence): add ConvergenceOptions::alpha damping field

Adds an EP damping coefficient defaulting to 1.0 (undamped). Will be
read by run_chain in a follow-up commit. By itself this commit changes
no behavior — existing constructors using ..Default::default() pick up
the new field automatically.

src/convergence.rs

@@ -8,6 +8,11 @@ use smallvec::SmallVec;
 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]`.
+    pub alpha: f64,
 }
 
 impl Default for ConvergenceOptions {
@@ -15,6 +20,7 @@ impl Default for ConvergenceOptions {
         Self {
             max_iter: crate::ITERATIONS,
             epsilon: crate::EPSILON,
+            alpha: 1.0,
         }
     }
 }
@@ -29,3 +35,14 @@ pub struct ConvergenceReport {
     pub per_iteration_time: SmallVec<[Duration; 32]>,
     pub slices_skipped: usize,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_alpha_is_one_for_undamped_behavior() {
+        let opts = ConvergenceOptions::default();
+        assert_eq!(opts.alpha, 1.0);
+    }
+}

src/history.rs

@@ -1368,6 +1368,7 @@ mod tests {
         h.convergence = ConvergenceOptions {
             max_iter: 11,
             epsilon: EPSILON,
+            alpha: 1.0,
         };
         h.converge().unwrap();
 
@@ -1685,6 +1686,7 @@ mod tests {
             .convergence(ConvergenceOptions {
                 max_iter: 30,
                 epsilon: 1e-6,
+                alpha: 1.0,
             })
             .build();