refactor(api): rename Player to Rating

The struct holds prior/beta/drift — a rating configuration, not a
person. The person-with-temporal-state is the Competitor (renamed in
the next task). Resolves Player/Agent ambiguity.

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

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib.rs

@@ -16,7 +16,7 @@ pub mod gaussian;
 mod history;
 mod key_table;
 mod matrix;
-pub mod player;
+mod rating;
 pub(crate) mod schedule;
 pub mod storage;
 
@@ -27,7 +27,7 @@ pub use gaussian::Gaussian;
 pub use history::History;
 pub use key_table::KeyTable;
 use matrix::Matrix;
-pub use player::Player;
+pub use rating::Rating;
 pub use schedule::ScheduleReport;
 
 pub const BETA: f64 = 1.0;