feat(api): add record_winner, record_draw, intern, lookup on History

Spec Section 4 "three-tier event ingestion" tier 2: one-off match
convenience. Spec open question 3: expose Index + intern/lookup for
power users.

History and HistoryBuilder gain a 4th generic parameter
K: Eq + Hash + Clone = &'static str. The default ensures existing
tests using Index-based add_events compile unchanged.

History internally owns a KeyTable<K>. intern(&Q) creates or returns
an Index for the given key; lookup(&Q) returns Option<Index> without
creating. record_winner and record_draw are thin 1v1 wrappers around
the internal add_events_with_prior.

Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/record_winner.rs

@@ -0,0 +1,54 @@
+use trueskill_tt::{ConstantDrift, ConvergenceOptions, History};
+
+#[test]
+fn record_winner_builds_history() {
+    let mut h = History::builder()
+        .mu(25.0)
+        .sigma(25.0 / 3.0)
+        .beta(25.0 / 6.0)
+        .drift(ConstantDrift(25.0 / 300.0))
+        .convergence(ConvergenceOptions {
+            max_iter: 30,
+            epsilon: 1e-6,
+        })
+        .build();
+
+    h.record_winner(&"alice", &"bob", 1).unwrap();
+    h.converge().unwrap();
+
+    let a_idx = h.lookup(&"alice").unwrap();
+    let b_idx = h.lookup(&"bob").unwrap();
+
+    assert_ne!(a_idx, b_idx);
+}
+
+#[test]
+fn intern_is_idempotent() {
+    let mut h: History = History::builder().build();
+    let a1 = h.intern(&"alice");
+    let a2 = h.intern(&"alice");
+    assert_eq!(a1, a2);
+}
+
+#[test]
+fn lookup_returns_none_for_missing() {
+    let h: History = History::builder().build();
+    assert!(h.lookup(&"nobody").is_none());
+}
+
+#[test]
+fn record_draw_with_p_draw_set() {
+    let mut h = History::builder()
+        .mu(25.0)
+        .sigma(25.0 / 3.0)
+        .beta(25.0 / 6.0)
+        .drift(ConstantDrift(25.0 / 300.0))
+        .p_draw(0.25)
+        .build();
+
+    h.record_draw(&"alice", &"bob", 1).unwrap();
+    h.converge().unwrap();
+
+    assert!(h.lookup(&"alice").is_some());
+    assert!(h.lookup(&"bob").is_some());
+}