feat(history): parallel log_evidence with deterministic sum

Per-slice log_evidence contribution computed in parallel under
--features rayon; final reduction is sequential .into_iter().sum()
on Vec<f64>, preserving slice order so the sum is bit-identical to
the sequential T2 baseline.

Essential for the T3 acceptance criterion of identical posteriors
across RAYON_NUM_THREADS values.

Part of T3.

src/history.rs

@@ -332,10 +332,23 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     }
 
     pub(crate) fn log_evidence_internal(&mut self, forward: bool, targets: &[Index]) -> f64 {
-        self.time_slices
-            .iter()
-            .map(|ts| ts.log_evidence(self.online, targets, forward, &self.agents))
-            .sum()
+        #[cfg(feature = "rayon")]
+        {
+            use rayon::prelude::*;
+            let per_slice: Vec<f64> = self
+                .time_slices
+                .par_iter()
+                .map(|ts| ts.log_evidence(self.online, targets, forward, &self.agents))
+                .collect();
+            per_slice.into_iter().sum()
+        }
+        #[cfg(not(feature = "rayon"))]
+        {
+            self.time_slices
+                .iter()
+                .map(|ts| ts.log_evidence(self.online, targets, forward, &self.agents))
+                .sum()
+        }
     }
 
     /// Total log-evidence across the history.