feat(api): add fluent history.event(t).team(...).commit() builder

Third tier of the ingestion API (spec Section 4). Powers one-off
events with irregular shapes where neither record_winner (too
simple) nor typed add_events (too verbose) fits cleanly.

EventBuilder accumulates teams, weights, and outcome. Supports:
- .team([keys]) — add a team
- .weights([w..]) — per-member weights on the most-recently-added team
- .ranking([ranks]) — explicit per-team ranks
- .winner(i) — convenience: team i wins, others tied
- .draw() — all teams tied
- .commit() — finalize into an Event<T, K> and delegate to add_events

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/api_shape.rs

@@ -82,3 +82,63 @@ fn add_events_rejects_mismatched_outcome_ranks() {
     let err = h.add_events(events).unwrap_err();
     assert!(matches!(err, InferenceError::MismatchedShape { .. }));
 }
+
+#[test]
+fn fluent_event_builder_basic() {
+    let mut h = History::builder()
+        .mu(25.0)
+        .sigma(25.0 / 3.0)
+        .beta(25.0 / 6.0)
+        .p_draw(0.0)
+        .build();
+
+    h.event(1)
+        .team(["alice", "bob"])
+        .weights([1.0, 0.7])
+        .team(["carol"])
+        .ranking([1, 0])
+        .commit()
+        .unwrap();
+
+    let report = h.converge().unwrap();
+    assert!(report.converged);
+    assert!(h.lookup(&"alice").is_some());
+    assert!(h.lookup(&"bob").is_some());
+    assert!(h.lookup(&"carol").is_some());
+}
+
+#[test]
+fn fluent_event_builder_winner_convenience() {
+    let mut h = History::builder()
+        .mu(25.0)
+        .sigma(25.0 / 3.0)
+        .beta(25.0 / 6.0)
+        .p_draw(0.0)
+        .build();
+
+    h.event(1)
+        .team(["alice"])
+        .team(["bob"])
+        .winner(0)
+        .commit()
+        .unwrap();
+    h.converge().unwrap();
+}
+
+#[test]
+fn fluent_event_builder_draw() {
+    let mut h = History::builder()
+        .mu(25.0)
+        .sigma(25.0 / 3.0)
+        .beta(25.0 / 6.0)
+        .p_draw(0.25)
+        .build();
+
+    h.event(1)
+        .team(["alice"])
+        .team(["bob"])
+        .draw()
+        .commit()
+        .unwrap();
+    h.converge().unwrap();
+}