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

src/history.rs

@@ -3053,8 +3053,8 @@ mod tests {
 
     use super::*;
     use crate::{
-        ConstantDrift, EPSILON, Event, Game, Gaussian, Member, Outcome, P_DRAW, Team,
-        arena::ScratchArena,
+        ConstantDrift, EPSILON, Event, Gaussian, Member, Outcome, P_DRAW, Team,
+        arena::ScratchArena, game::GameRef,
     };
 
     /// #17: a slice's footprint must be O(competitors in the slice), not
@@ -3170,7 +3170,7 @@ mod tests {
         let observed = h.time_slices[1].skills.get(a).unwrap().posterior();
 
         let w = [vec![1.0], vec![1.0]];
-        let p = Game::ranked_with_arena(
+        let p = GameRef::ranked_with_arena(
             h.time_slices[1].events[0].within_priors(
                 false,
                 &h.time_slices[1].skills,