Posterior
Stateless conjugate posterior over a univariate likelihood, parameterised by the sufficient-statistic snapshot R. A Posterior is a pure (snapshot, rng) -> sample function: no priors, no per-arm state, no update path. Arm lifecycle, value encoding, and prior seeding all live in Arm.
Sealed + @Serializable so a (arm, posterior) Thompson configuration is wire-portable via the kumulant/skema serialization convention. The posterior assumes the snapshot already incorporates whatever pseudo-counts the caller wants; with a seeded snapshot the empty-snapshot edge case can't arise.