refactor(api): rename Batch to TimeSlice

TimeSlice says what it is: every event sharing one timestamp. The
History field .batches is renamed to .time_slices. Local variables
named `batch` referring to TimeSlice instances are renamed to
`time_slice`.

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

src/lib.rs

@@ -6,7 +6,8 @@ use std::{
 #[cfg(feature = "approx")]
 mod approx;
 pub(crate) mod arena;
-pub mod batch;
+mod time_slice;
+pub use time_slice::TimeSlice;
 mod competitor;
 pub mod drift;
 mod error;