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/key_table.rs

@@ -0,0 +1,72 @@
+use std::{
+    borrow::{Borrow, ToOwned},
+    collections::HashMap,
+    hash::Hash,
+};
+
+use crate::Index;
+
+/// Maps user keys to internal `Index` handles.
+///
+/// Renamed from the former `IndexMap` to avoid colliding with the `indexmap`
+/// crate. Power users can promote `&K` to `Index` via `get_or_create` and
+/// skip the lookup on subsequent hot-path calls.
+#[derive(Debug)]
+pub struct KeyTable<K>(HashMap<K, Index>);
+
+impl<K> KeyTable<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()
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl<K> Default for KeyTable<K>
+where
+    K: Eq + Hash,
+{
+    fn default() -> Self {
+        KeyTable::new()
+    }
+}