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

src/time_slice.rs

@@ -277,8 +277,8 @@ impl<T: Time> TimeSlice<T> {
     pub fn add_events<D: Drift<T>>(
         &mut self,
         composition: Vec<Vec<Vec<Index>>>,
-        results: Vec<Vec<f64>>,
-        weights: Vec<Vec<Vec<f64>>>,
+        results: Option<Vec<Vec<f64>>>,
+        weights: Option<Vec<Vec<Vec<f64>>>>,
         kinds: Vec<EventKind>,
         agents: &CompetitorStore<T, D>,
     ) {
@@ -328,22 +328,21 @@ impl<T: Time> TimeSlice<T> {
 
                     Team {
                         items,
-                        output: if results.is_empty() {
-                            (event.len() - (t + 1)) as f64
-                        } else {
-                            results[e][t]
+                        output: match &results {
+                            Some(results) => results[e][t],
+                            // No explicit result: rank by position, first team best.
+                            None => (event.len() - (t + 1)) as f64,
                         },
                     }
                 })
                 .collect::<Vec<_>>();
 
-            let weights = if weights.is_empty() {
-                teams
+            let weights = match &weights {
+                Some(weights) => weights[e].clone(),
+                None => teams
                     .iter()
                     .map(|team| vec![1.0; team.items.len()])
-                    .collect::<Vec<_>>()
-            } else {
-                weights[e].clone()
+                    .collect::<Vec<_>>(),
             };
 
             Event {
@@ -828,8 +827,8 @@ mod tests {
                 vec![vec![c], vec![d]],
                 vec![vec![e], vec![f]],
             ],
-            vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]],
-            vec![],
+            Some(vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]]),
+            None,
             vec![EventKind::Ranked; 3],
             &agents,
         );
@@ -905,8 +904,8 @@ mod tests {
                 vec![vec![a], vec![c]],
                 vec![vec![b], vec![c]],
             ],
-            vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]],
-            vec![],
+            Some(vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]]),
+            None,
             vec![EventKind::Ranked; 3],
             &agents,
         );
@@ -985,8 +984,8 @@ mod tests {
                 vec![vec![a], vec![c]],
                 vec![vec![b], vec![c]],
             ],
-            vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]],
-            vec![],
+            Some(vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]]),
+            None,
             vec![EventKind::Ranked; 3],
             &agents,
         );
@@ -1017,8 +1016,8 @@ mod tests {
                 vec![vec![a], vec![c]],
                 vec![vec![b], vec![c]],
             ],
-            vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]],
-            vec![],
+            Some(vec![vec![1.0, 0.0], vec![0.0, 1.0], vec![1.0, 0.0]]),
+            None,
             vec![EventKind::Ranked; 3],
             &agents,
         );
@@ -1088,8 +1087,8 @@ mod tests {
                 vec![vec![c], vec![d]],
                 vec![vec![a], vec![c]],
             ],
-            vec![vec![1.0, 0.0], vec![1.0, 0.0], vec![1.0, 0.0]],
-            vec![],
+            Some(vec![vec![1.0, 0.0], vec![1.0, 0.0], vec![1.0, 0.0]]),
+            None,
             vec![EventKind::Ranked; 3],
             &agents,
         );