Replaces the old nested Vec<Vec<Vec<_>>> event description on the
public API boundary. Member<K>::from(K) enables ergonomic literal
lists. Member::with_weight / with_prior are builder methods for the
optional per-event overrides.
Fully additive — no existing call sites updated. Consumed by
History::add_events(iter) in Task 15.
Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.
