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.

examples/atp.rs

@@ -1,6 +1,6 @@
 use plotters::prelude::*;
 use time::{Date, Month};
-use trueskill_tt::{History, IndexMap};
+use trueskill_tt::{History, KeyTable};
 
 fn main() {
     let mut csv = csv::Reader::open("examples/atp.csv").unwrap();
@@ -12,7 +12,7 @@ fn main() {
     let from = Date::from_calendar_date(1900, Month::January, 1).unwrap();
     let time_format = time::format_description::parse("[year]-[month]-[day]").unwrap();
 
-    let mut index_map = IndexMap::new();
+    let mut index_map = KeyTable::new();
 
     for row in csv.records() {
         if &row["double"] == "t" {