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/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();