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>

src/lib.rs

@@ -14,6 +14,7 @@ mod convergence;
 pub mod drift;
 mod error;
 mod event;
+mod event_builder;
 pub(crate) mod factor;
 mod game;
 pub mod gaussian;
@@ -31,6 +32,7 @@ pub use convergence::{ConvergenceOptions, ConvergenceReport};
 pub use drift::{ConstantDrift, Drift};
 pub use error::InferenceError;
 pub use event::{Event, Member, Team};
+pub use event_builder::EventBuilder;
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;