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).

benches/batch.rs

@@ -1,7 +1,7 @@
 use criterion::{Criterion, criterion_group, criterion_main};
 use trueskill_tt::{
-    BETA, Competitor, EventKind, GAMMA, KeyTable, MU, P_DRAW, Rating, SIGMA, TimeSlice,
-    drift::ConstantDrift, gaussian::Gaussian, storage::CompetitorStore,
+    BETA, Competitor, ConvergenceOptions, EventKind, GAMMA, KeyTable, MU, P_DRAW, Rating, SIGMA,
+    TimeSlice, drift::ConstantDrift, gaussian::Gaussian, storage::CompetitorStore,
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
@@ -35,7 +35,7 @@ fn criterion_benchmark(criterion: &mut Criterion) {
 
     let kinds = vec![EventKind::Ranked; composition.len()];
 
-    let mut time_slice = TimeSlice::new(1, P_DRAW);
+    let mut time_slice = TimeSlice::new(1, P_DRAW, ConvergenceOptions::default());
     time_slice.add_events(composition, results, weights, kinds, &agents);
 
     criterion.bench_function("Batch::iteration", |b| {