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,