perf(time-slice): restore sequential direct-write path under cfg(not(feature = "rayon"))

The compute/apply split introduced in 3680c54 was always active — the
sequential build paid EventOutput heap-alloc overhead even without
rayon, regressing Batch::iteration from 23.46 µs to 33.79 µs (+44%).

This commit makes the split feature-gated: under cfg(feature = "rayon")
the compute/apply pattern stays (needed for par_iter); under
cfg(not(feature = "rayon")) events update SkillStore inline via
Event::iteration_direct, matching the T2 performance profile.

EventOutput, Event::compute, and Event::apply_output are now
cfg(feature = "rayon")-only. TimeSlice::sweep_color_groups has two
cfg-gated implementations sharing the same signature.

Sequential restored to 23.29 µs; parallel 34.31 µs (small-workload
overhead expected — rayon threadpool amortizes at larger scales).

Part of T3.

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

src/time_slice.rs

@@ -85,13 +85,13 @@ pub(crate) struct Event {
 }
 
 /// Output of a single event's inference pass — ready to apply back to shared state.
+///
+/// Only used under the rayon feature to decouple the parallel compute phase from
+/// the sequential apply phase. Without rayon the direct-write path is used instead.
+#[cfg(feature = "rayon")]
 struct EventOutput {
-    /// New per-team/per-item likelihoods (same shape as `event.teams`).
     likelihoods: Vec<Vec<Gaussian>>,
     evidence: f64,
-    /// (agent index, new skill likelihood) pairs for the sequential apply step
-    /// that updates `SkillStore`. Computed while holding `&SkillStore` so the
-    /// caller only needs `&mut SkillStore` when writing back.
     skill_updates: Vec<(Index, Gaussian)>,
 }
 
@@ -130,6 +130,10 @@ impl Event {
     /// Compute the inference update for this event, returning an `EventOutput`
     /// that describes the mutations to apply. Takes only shared references so
     /// it can run inside a parallel closure.
+    ///
+    /// Only compiled under the rayon feature; the sequential path uses
+    /// `iteration_direct` instead to avoid `EventOutput` heap allocation.
+    #[cfg(feature = "rayon")]
     fn compute<T: Time, D: Drift<T>>(
         &self,
         skills: &SkillStore,
@@ -141,7 +145,6 @@ impl Event {
         let result = self.outputs();
         let g = Game::ranked_with_arena(teams, &result, &self.weights, p_draw, &mut arena);
 
-        // Pre-compute new skill likelihoods while we still hold &skills.
         let mut skill_updates: Vec<(Index, Gaussian)> = Vec::new();
         for (t, team) in self.teams.iter().enumerate() {
             for (i, item) in team.items.iter().enumerate() {
@@ -163,6 +166,7 @@ impl Event {
     /// Apply an `EventOutput` back onto this event's mutable item likelihoods
     /// and evidence. The `SkillStore` updates are applied separately by the
     /// caller to avoid conflicting borrows.
+    #[cfg(feature = "rayon")]
     fn apply_output(&mut self, output: &EventOutput) {
         self.evidence = output.evidence;
         for (t, team) in self.teams.iter_mut().enumerate() {
@@ -171,6 +175,33 @@ impl Event {
             }
         }
     }
+
+    /// Direct in-loop update: mutates self and `skills` inline with no
+    /// intermediate allocation. Used by the sequential (no rayon) sweep path
+    /// to match T2 performance.
+    #[cfg(not(feature = "rayon"))]
+    fn iteration_direct<T: Time, D: Drift<T>>(
+        &mut self,
+        skills: &mut SkillStore,
+        agents: &CompetitorStore<T, D>,
+        p_draw: f64,
+        arena: &mut ScratchArena,
+    ) {
+        let teams = self.within_priors(false, false, skills, agents);
+        let result = self.outputs();
+        let g = Game::ranked_with_arena(teams, &result, &self.weights, p_draw, arena);
+
+        for (t, team) in self.teams.iter_mut().enumerate() {
+            for (i, item) in team.items.iter_mut().enumerate() {
+                let old_likelihood = skills.get(item.agent).unwrap().likelihood;
+                let new_likelihood = (old_likelihood / item.likelihood) * g.likelihoods[t][i];
+                skills.get_mut(item.agent).unwrap().likelihood = new_likelihood;
+                item.likelihood = g.likelihoods[t][i];
+            }
+        }
+
+        self.evidence = g.evidence;
+    }
 }
 
 #[derive(Debug)]
@@ -355,40 +386,24 @@ impl<T: Time> TimeSlice<T> {
 
     /// Full event sweep using the color-group partition. Colors are processed
     /// sequentially; within each color the inner loop is parallel under rayon.
+    #[cfg(feature = "rayon")]
     fn sweep_color_groups<D: Drift<T>>(&mut self, agents: &CompetitorStore<T, D>) {
-        // We need &self.skills (immutable) and &mut self.events (mutable) at the
-        // same time. Rust allows this because they are distinct struct fields.
-        // The parallel closure captures &self.skills and &self.p_draw by shared
-        // ref; it returns owned EventOutput values that we apply sequentially.
+        use rayon::prelude::*;
+
         for color_idx in 0..self.color_groups.groups.len() {
             if self.color_groups.groups[color_idx].is_empty() {
                 continue;
             }
             let range = self.color_groups.color_range(color_idx);
 
-            // Compute phase — parallel under rayon, sequential otherwise.
-            // Borrows: &self.skills and &agents are shared refs captured by the closure;
-            // &mut self.events[range] is the mutable slice for par_iter_mut.
             let p_draw = self.p_draw;
             let skills: &SkillStore = &self.skills;
 
-            #[cfg(feature = "rayon")]
-            let outputs: Vec<EventOutput> = {
-                use rayon::prelude::*;
-                self.events[range.clone()]
-                    .par_iter()
-                    .map(|ev| ev.compute(skills, agents, p_draw))
-                    .collect()
-            };
-
-            #[cfg(not(feature = "rayon"))]
             let outputs: Vec<EventOutput> = self.events[range.clone()]
-                .iter()
+                .par_iter()
                 .map(|ev| ev.compute(skills, agents, p_draw))
                 .collect();
 
-            // Apply phase — sequential: write skill likelihoods back to self.skills,
-            // then update per-event item likelihoods and evidence.
             for (ev, output) in self.events[range].iter_mut().zip(outputs.iter()) {
                 for &(agent, new_skill_lhood) in &output.skill_updates {
                     self.skills.get_mut(agent).unwrap().likelihood = new_skill_lhood;
@@ -398,6 +413,27 @@ impl<T: Time> TimeSlice<T> {
         }
     }
 
+    /// Full event sweep using the color-group partition, sequential direct-write path.
+    /// Events within each color group are updated inline — no EventOutput allocation —
+    /// matching the T2 performance profile.
+    #[cfg(not(feature = "rayon"))]
+    fn sweep_color_groups<D: Drift<T>>(&mut self, agents: &CompetitorStore<T, D>) {
+        for color_idx in 0..self.color_groups.groups.len() {
+            if self.color_groups.groups[color_idx].is_empty() {
+                continue;
+            }
+            let range = self.color_groups.color_range(color_idx);
+
+            // Borrow self.events as a mutable slice for this color range.
+            // self.skills and self.arena are separate fields — disjoint borrows are
+            // allowed within a single method body.
+            let p_draw = self.p_draw;
+            for ev in &mut self.events[range] {
+                ev.iteration_direct(&mut self.skills, agents, p_draw, &mut self.arena);
+            }
+        }
+    }
+
     #[allow(dead_code)]
     pub(crate) fn convergence<D: Drift<T>>(&mut self, agents: &CompetitorStore<T, D>) -> usize {
         let epsilon = 1e-6;