feat(api): add Event<T, K>, Team<K>, Member<K> typed event description

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.

src/lib.rs

@@ -12,6 +12,7 @@ pub use time_slice::TimeSlice;
 mod competitor;
 pub mod drift;
 mod error;
+mod event;
 pub(crate) mod factor;
 mod game;
 pub mod gaussian;
@@ -26,6 +27,7 @@ pub mod storage;
 pub use competitor::Competitor;
 pub use drift::{ConstantDrift, Drift};
 pub use error::InferenceError;
+pub use event::{Event, Member, Team};
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;