New public types:
- ConvergenceOptions { max_iter, epsilon } — config for the loop
- ConvergenceReport { iterations, final_step, log_evidence, converged,
per_iteration_time, slices_skipped } — post-hoc summary
History and HistoryBuilder gain a third generic parameter
O: Observer<T> = NullObserver. Builder methods:
- .convergence(opts) sets the ConvergenceOptions
- .observer(o) plugs in an Observer (reshapes the builder's O param)
History::converge() runs the existing iteration loop driven by the
stored opts, emits observer callbacks on each iteration end and on
completion, and returns Result<ConvergenceReport, InferenceError>.
The old convergence(iters, eps, verbose) stays — gets removed in
Task 20 after tests are translated.
Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.
