refactor(api): generify Drift, Rating, Competitor, TimeSlice, CompetitorStore, History over T: Time

Drift now takes &T -> &T and is generic over the time axis. Untimed
impls return elapsed=0. ConstantDrift impl covers all T via the Time
trait. An additional variance_for_elapsed(i64) method on the trait
serves callers that work with the pre-cached i64 elapsed count.

Competitor.last_time moves from i64 with MIN sentinel to Option<T>
with None sentinel. receive(&T) computes variance from last_time
dynamically; receive_for_elapsed(i64) uses a pre-cached elapsed count
(needed in convergence sweeps where last_time has already advanced).

TimeSlice.time changes from i64 to T. compute_elapsed is now generic
over T and takes Option<&T> for the last-seen time. new_forward_info
uses receive_for_elapsed to preserve the cached elapsed during sweeps.

History<D> becomes History<T, D>; HistoryBuilder<D> becomes
HistoryBuilder<T, D>; Game<D> becomes Game<T, D>. Defaults keep
existing call sites compiling with zero changes: T = i64,
D = ConstantDrift.

add_events / add_events_with_prior stay on impl History<i64, D> since
times: Vec<i64> is i64-specific (Task 8 will generalise this).

In !self.time mode the old i64::MAX sentinel guaranteed elapsed=1 for
every slice transition regardless of time gaps. Replaced by advancing
all previously-seen agents' last_time to Some(current_slice_time) at
the end of each slice; this preserves elapsed=1 between adjacent
slices in sequential-integer untimed mode.

The time: bool field on History and .time(bool) on HistoryBuilder are
NOT removed by this task — deferred to Task 8 so this commit is
purely a type-level generification.

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

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

benches/batch.rs

@@ -11,7 +11,7 @@ fn criterion_benchmark(criterion: &mut Criterion) {
     let b = index_map.get_or_create("b");
     let c = index_map.get_or_create("c");
 
-    let mut agents: CompetitorStore<ConstantDrift> = CompetitorStore::new();
+    let mut agents: CompetitorStore<i64, ConstantDrift> = CompetitorStore::new();
 
     for agent in [a, b, c] {
         agents.insert(