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>

benches/batch.rs

@@ -5,11 +5,11 @@ use trueskill_tt::{
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
-    let mut index = KeyTable::new();
+    let mut index_map = KeyTable::new();
 
-    let a = index.get_or_create("a");
-    let b = index.get_or_create("b");
-    let c = index.get_or_create("c");
+    let a = index_map.get_or_create("a");
+    let b = index_map.get_or_create("b");
+    let c = index_map.get_or_create("c");
 
     let mut agents: AgentStore<ConstantDrift> = AgentStore::new();