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.

src/lib.rs

@@ -1,9 +1,6 @@
 use std::{
-    borrow::{Borrow, ToOwned},
     cmp::Reverse,
-    collections::HashMap,
     f64::consts::{FRAC_1_SQRT_2, FRAC_2_SQRT_PI, SQRT_2},
-    hash::Hash,
 };
 
 pub mod agent;
@@ -17,6 +14,7 @@ pub(crate) mod factor;
 mod game;
 pub mod gaussian;
 mod history;
+pub mod key_table;
 mod matrix;
 pub mod player;
 pub(crate) mod schedule;
@@ -27,6 +25,7 @@ pub use error::InferenceError;
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;
+pub use key_table::KeyTable;
 use matrix::Matrix;
 pub use player::Player;
 pub use schedule::ScheduleReport;
@@ -54,59 +53,6 @@ impl From<usize> for Index {
     }
 }
 
-pub struct IndexMap<K>(HashMap<K, Index>);
-
-impl<K> IndexMap<K>
-where
-    K: Eq + Hash,
-{
-    pub fn new() -> Self {
-        Self(HashMap::new())
-    }
-
-    pub fn get<Q: ?Sized + Hash + Eq + ToOwned<Owned = K>>(&self, k: &Q) -> Option<Index>
-    where
-        K: Borrow<Q>,
-    {
-        self.0.get(k).cloned()
-    }
-
-    pub fn get_or_create<Q: ?Sized + Hash + Eq + ToOwned<Owned = K>>(&mut self, k: &Q) -> Index
-    where
-        K: Borrow<Q>,
-    {
-        if let Some(idx) = self.0.get(k) {
-            *idx
-        } else {
-            let idx = Index::from(self.0.len());
-
-            self.0.insert(k.to_owned(), idx);
-
-            idx
-        }
-    }
-
-    pub fn key(&self, idx: Index) -> Option<&K> {
-        self.0
-            .iter()
-            .find(|&(_, value)| *value == idx)
-            .map(|(key, _)| key)
-    }
-
-    pub fn keys(&self) -> impl Iterator<Item = &K> {
-        self.0.keys()
-    }
-}
-
-impl<K> Default for IndexMap<K>
-where
-    K: Eq + Hash,
-{
-    fn default() -> Self {
-        IndexMap::new()
-    }
-}
-
 fn erfc(x: f64) -> f64 {
     let z = x.abs();
     let t = 1.0 / (1.0 + z / 2.0);