refactor(time_slice): add convergence field, rename iterate_to_convergence

TimeSlice<T> gains a pub(crate) convergence: ConvergenceOptions field
set at construction. TimeSlice::new now takes it as a third parameter
(breaking change to the pub constructor, acceptable in 0.1.x).
History::add_events_with_prior passes self.convergence so the propagated
value reaches every TimeSlice. The pre-existing convergence-the-method
is renamed to iterate_to_convergence to disambiguate from the new
convergence-the-field.

The field is wired but not yet read by inference -- the three
Game::*_with_arena callsites in time_slice.rs still hardcode
ConvergenceOptions::default(). Task 2 changes that. Bit-equal because
the propagated value equals the hardcoded value end-to-end.

Also updated benches/batch.rs which has a fourth TimeSlice::new
callsite (not enumerated in the plan -- only src/ files were).

src/history.rs

@@ -594,7 +594,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                     agent.message = time_slice.forward_prior_out(&agent_idx);
                 }
             } else {
-                let mut time_slice = TimeSlice::new(t, self.p_draw);
+                let mut time_slice = TimeSlice::new(t, self.p_draw, self.convergence);
                 time_slice.add_events(composition, results, weights, kinds_chunk, &self.agents);
 
                 self.time_slices.insert(k, time_slice);