feat(api): add Time trait with Untimed and i64 impls

Foundation for generic History time axis. Untimed is the ZST case
(no drift across slices); i64 is the standard timestamp case.
Additional impls (time::OffsetDateTime, chrono) can be added behind
feature flags in follow-up work.

The trait is not yet wired into History — that happens in Task 7
along with generifying Drift over T.

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

src/lib.rs

@@ -6,6 +6,7 @@ use std::{
 #[cfg(feature = "approx")]
 mod approx;
 pub(crate) mod arena;
+mod time;
 mod time_slice;
 pub use time_slice::TimeSlice;
 mod competitor;
@@ -31,6 +32,7 @@ pub use key_table::KeyTable;
 use matrix::Matrix;
 pub use rating::Rating;
 pub use schedule::ScheduleReport;
+pub use time::{Time, Untimed};
 
 pub const BETA: f64 = 1.0;
 pub const MU: f64 = 0.0;