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/event.rs

@@ -0,0 +1,132 @@
+//! Typed event description for bulk ingestion.
+//!
+//! `Event<T, K>` is the new public event shape (spec Section 4). Replaces
+//! the nested `Vec<Vec<Vec<Index>>>`, `Vec<Vec<f64>>`, `Vec<Vec<Vec<f64>>>`
+//! that the old `add_events_with_prior` took.
+
+use smallvec::SmallVec;
+
+use crate::{gaussian::Gaussian, outcome::Outcome, time::Time};
+
+/// A single match at time `time` involving some number of teams.
+#[derive(Clone, Debug)]
+pub struct Event<T: Time, K> {
+    pub time: T,
+    pub teams: SmallVec<[Team<K>; 4]>,
+    pub outcome: Outcome,
+}
+
+/// A team: list of members competing together.
+#[derive(Clone, Debug)]
+pub struct Team<K> {
+    pub members: SmallVec<[Member<K>; 4]>,
+}
+
+impl<K> Team<K> {
+    pub fn new() -> Self {
+        Self {
+            members: SmallVec::new(),
+        }
+    }
+
+    pub fn with_members<I: IntoIterator<Item = Member<K>>>(members: I) -> Self {
+        Self {
+            members: members.into_iter().collect(),
+        }
+    }
+}
+
+impl<K> Default for Team<K> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// One member of a team, identified by user key `K`.
+///
+/// `weight` defaults to 1.0; a per-event `prior` can override the competitor's
+/// current skill estimate for this event only.
+#[derive(Clone, Debug)]
+pub struct Member<K> {
+    pub key: K,
+    pub weight: f64,
+    pub prior: Option<Gaussian>,
+}
+
+impl<K> Member<K> {
+    pub fn new(key: K) -> Self {
+        Self {
+            key,
+            weight: 1.0,
+            prior: None,
+        }
+    }
+
+    pub fn with_weight(mut self, weight: f64) -> Self {
+        self.weight = weight;
+        self
+    }
+
+    pub fn with_prior(mut self, prior: Gaussian) -> Self {
+        self.prior = Some(prior);
+        self
+    }
+}
+
+/// Convenience: a member is a user key with default weight 1.0 and no prior.
+impl<K> From<K> for Member<K> {
+    fn from(key: K) -> Self {
+        Self::new(key)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::Outcome;
+
+    #[test]
+    fn member_new_has_unit_weight_no_prior() {
+        let m = Member::new("alice");
+        assert_eq!(m.key, "alice");
+        assert_eq!(m.weight, 1.0);
+        assert!(m.prior.is_none());
+    }
+
+    #[test]
+    fn member_builder_methods_chain() {
+        let m = Member::new("alice")
+            .with_weight(0.5)
+            .with_prior(Gaussian::from_ms(20.0, 5.0));
+        assert_eq!(m.weight, 0.5);
+        assert!(m.prior.is_some());
+    }
+
+    #[test]
+    fn member_from_key() {
+        let m: Member<&str> = "bob".into();
+        assert_eq!(m.key, "bob");
+        assert_eq!(m.weight, 1.0);
+    }
+
+    #[test]
+    fn team_with_members_collects() {
+        let t: Team<&str> = Team::with_members([Member::new("a"), Member::new("b")]);
+        assert_eq!(t.members.len(), 2);
+    }
+
+    #[test]
+    fn event_construction() {
+        use smallvec::smallvec;
+        let e: Event<i64, &str> = Event {
+            time: 1,
+            teams: smallvec![
+                Team::with_members([Member::new("a")]),
+                Team::with_members([Member::new("b")]),
+            ],
+            outcome: Outcome::winner(0, 2),
+        };
+        assert_eq!(e.teams.len(), 2);
+        assert_eq!(e.time, 1);
+    }
+}

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;