refactor(api): rename IndexMap to KeyTable

The former name collided with the popular indexmap crate. KeyTable
lives in its own module. Public API unchanged beyond the rename.

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

benches/batch.rs

@@ -1,11 +1,11 @@
 use criterion::{Criterion, criterion_group, criterion_main};
 use trueskill_tt::{
-    BETA, GAMMA, IndexMap, MU, P_DRAW, SIGMA, agent::Agent, batch::Batch, drift::ConstantDrift,
+    BETA, GAMMA, KeyTable, MU, P_DRAW, SIGMA, agent::Agent, batch::Batch, drift::ConstantDrift,
     gaussian::Gaussian, player::Player, storage::AgentStore,
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
-    let mut index = IndexMap::new();
+    let mut index = KeyTable::new();
 
     let a = index.get_or_create("a");
     let b = index.get_or_create("b");