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.

Cargo.toml

@@ -16,6 +16,7 @@ harness = false
 
 [dependencies]
 approx = { version = "0.5.1", optional = true }
+smallvec = "1"
 
 [dev-dependencies]
 criterion = "0.5"