feat!: Game is the type you get, and one_v_one returns one

`lib.rs` advertised `Game` in "Core types" as "one match in isolation".
It had no public constructor: every `Game::*` returned `OwnedGame`, so
`let g: Game = Game::ranked(..)?` did not compile.

Names swapped. The public type is the owned one — `Game<T, D>`, no
lifetime — and the borrowing form is `pub(crate) GameRef<'a, T, D>`,
which is what it always was: an implementation detail about whether the
result and weight slices are borrowed from `History`'s storage. That
distinction meant nothing to someone scoring one match, and it showed
the module's surface twice in rustdoc, since both types carried
`posteriors()` / `log_evidence()`.

`one_v_one` returned `(Gaussian, Gaussian)` while every sibling returned
a game, making it the one constructor you could not ask for
`log_evidence()`. It returns `Self` now; `.posteriors()` recovers the old
shape, and the test that covers it now also asserts the evidence of two
identical ratings is exactly `ln(0.5)`.

`ranked`, `scored` and `free_for_all` had `# Errors` as their entire
doc, so rustdoc's index rendered the error list as the summary. They
have summary lines, and `Game` has a worked example — it was advertised
as a core type with none anywhere in the crate.

Closes #69.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

tests/equivalence.rs

@@ -23,8 +23,10 @@ fn ts_rating(mu: f64, sigma: f64, beta: f64, gamma: f64) -> R {
 fn game_1v1_golden_matches_historical() {
     let a = ts_rating(25.0, 25.0 / 3.0, 25.0 / 6.0, 25.0 / 300.0);
     let b = ts_rating(25.0, 25.0 / 3.0, 25.0 / 6.0, 25.0 / 300.0);
-    let (a_post, b_post) =
-        Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &GameOptions::default()).unwrap();
+    let post = Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &GameOptions::default())
+        .unwrap()
+        .posteriors();
+    let (a_post, b_post) = (post[0][0], post[1][0]);
     // Historical golden from pre-T2 test_1vs1 (team 0 wins):
     assert_ulps_eq!(
         a_post,

tests/game.rs

@@ -32,10 +32,16 @@ fn game_ranked_1v1_golden() {
 fn game_one_v_one_shortcut() {
     let a = default_rating();
     let b = default_rating();
-    let (a_post, b_post) =
+    let game =
         Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &GameOptions::default()).unwrap();
+    let post = game.posteriors();
+    let (a_post, b_post) = (post[0][0], post[1][0]);
     assert!(a_post.mu() > 25.0);
     assert!(b_post.mu() < 25.0);
+
+    // It returns a game like every other constructor, so evidence is askable.
+    // Two identical ratings make either result equally likely.
+    assert!((game.log_evidence() - 0.5_f64.ln()).abs() < 1e-12);
 }
 
 #[test]
@@ -118,8 +124,10 @@ fn one_v_one_honours_the_draw_probability_it_is_given() {
         p_draw: 0.25,
         ..GameOptions::default()
     };
-    let (a_post, b_post) = Game::<i64, _>::one_v_one(&a, &b, Outcome::draw(2), &options)
-        .expect("a draw is representable once p_draw is positive");
+    let post = Game::<i64, _>::one_v_one(&a, &b, Outcome::draw(2), &options)
+        .expect("a draw is representable once p_draw is positive")
+        .posteriors();
+    let (a_post, b_post) = (post[0][0], post[1][0]);
 
     // A symmetric draw leaves the means alone and sharpens both sides.
     assert!((a_post.mu() - b_post.mu()).abs() < 1e-9);
@@ -135,8 +143,10 @@ fn one_v_one_honours_convergence_options() {
         convergence: ConvergenceOptions::default(),
         ..GameOptions::default()
     };
-    let (a_post, _) = Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &options).unwrap();
-    assert!(a_post.mu() > 25.0);
+    let post = Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &options)
+        .unwrap()
+        .posteriors();
+    assert!(post[0][0].mu() > 25.0);
 }
 
 /// `Game` is a public entry point that does not pass through `History`'s