Closes the gap between HistoryBuilder::convergence(opts) and the
within-game inference loop. TimeSlice gains a convergence field;
History passes self.convergence at construction; the three
Game::*_with_arena callsites in time_slice.rs read it. Also renames
TimeSlice::convergence the method (now iterate_to_convergence) to
disambiguate from the new field.
Pure plumbing — no new public API, no behavioral change for users on
default options. Makes Damped EP reachable through the History path.
