refactor!: replace emptiness-as-sentinel with Option for results and weights

Third of the five remaining items on #23.

`add_events_with_prior` and `TimeSlice::add_events` took `Vec<Vec<f64>>` and
`Vec<Vec<Vec<f64>>>` where an empty vec meant "not supplied" — so an empty
outer vec and a genuinely empty event list were the same value, and every
reader had to know which. Both are now `Option`, and the "not supplied"
branches read as `None` arms rather than `is_empty()` checks.

Two things fell out of the change that a sentinel would have hidden:

The tie pre-check iterated `results` directly. Under `Option` it needs
`.iter().flatten()`, which makes explicit that a `None` results list has no
ties to reject — previously an empty vec silently skipped the same loop and
looked identical to "checked, found nothing".

`MismatchedShape.got` could no longer be `results.len()`, because at the point
of the error there may be no vec to take a length from. It is now computed as
`map_or(0, Vec::len)` before the error is built.

MSRV note: my first draft used let-chains for the two validations. Those need
Rust 1.88 and this crate pins 1.85 — it compiled locally on 1.98 and would
have failed only in the MSRV CI job. Rewritten with `is_some_and`, and
verified by installing 1.85.0 and building against it rather than by assuming
the removal was complete.

Breaking: `TimeSlice::add_events` is public.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T5SYDExxL4vZgvunrcNSMc

benches/batch.rs

@@ -36,7 +36,7 @@ fn criterion_benchmark(criterion: &mut Criterion) {
     let kinds = vec![EventKind::Ranked; composition.len()];
 
     let mut time_slice = TimeSlice::new(1, P_DRAW, ConvergenceOptions::default());
-    time_slice.add_events(composition, results, weights, kinds, &agents);
+    time_slice.add_events(composition, Some(results), Some(weights), kinds, &agents);
 
     criterion.bench_function("Batch::iteration", |b| {
         b.iter(|| time_slice.iteration(0, &agents))