refactor(lib): make key_table module private; revert bench var rename

Address code review feedback from Task 2:
- key_table module doesn't need pub visibility; the KeyTable re-export
  at lib.rs root already exposes the only public type. Matches the
  error/history private-module pattern.
- Revert an incidental bench variable rename (index_map → index) that
  wasn't part of the task scope.

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

src/lib.rs

@@ -14,7 +14,7 @@ pub(crate) mod factor;
 mod game;
 pub mod gaussian;
 mod history;
-pub mod key_table;
+mod key_table;
 mod matrix;
 pub mod player;
 pub(crate) mod schedule;