feat(api): add Outcome enum with Ranked variant

Outcome::winner(i, n), Outcome::draw(n), Outcome::ranking(iter) are
the convenience constructors. Marked #[non_exhaustive] so Scored can
be added in T4 without breaking match exhaustiveness.

Adds smallvec = "1" as a direct dependency.

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

src/lib.rs

@@ -18,6 +18,7 @@ pub mod gaussian;
 mod history;
 mod key_table;
 mod matrix;
+mod outcome;
 mod rating;
 pub(crate) mod schedule;
 pub mod storage;
@@ -30,6 +31,7 @@ pub use gaussian::Gaussian;
 pub use history::History;
 pub use key_table::KeyTable;
 use matrix::Matrix;
+pub use outcome::Outcome;
 pub use rating::Rating;
 pub use schedule::ScheduleReport;
 pub use time::{Time, Untimed};