refactor!: remove the inert online flag

Skill.online was initialised to N_INF and assigned nowhere, so
HistoryBuilder::online(true) made every rating improper and log_evidence()
reported n * ln(0.5) — every game scored as a coin flip. The value is finite
and plausible, which is why it went unnoticed.

The default was false, so no existing result changes. A working replacement
lands next; a stored field cannot hold the quantity, because converge()
alternates sweeps and contaminates skill.forward with backward information
from the second iteration onward.

Also renames a test binding from ..._online to ..._forward: it passes the
forward flag, and the two senses being conflated is how this survived.

src/time_slice.rs

@@ -22,7 +22,6 @@ pub(crate) struct Skill {
     backward: Gaussian,
     likelihood: Gaussian,
     pub(crate) elapsed: i64,
-    pub(crate) online: Gaussian,
 }
 
 impl Skill {
@@ -38,7 +37,6 @@ impl Default for Skill {
             backward: N_INF,
             likelihood: N_INF,
             elapsed: 0,
-            online: N_INF,
         }
     }
 }
@@ -59,7 +57,6 @@ struct Item {
 impl Item {
     fn within_prior<T: Time, D: Drift<T>>(
         &self,
-        online: bool,
         forward: bool,
         skills: &SkillStore,
         agents: &CompetitorStore<T, D>,
@@ -67,9 +64,7 @@ impl Item {
         let r = &agents[self.agent].rating;
         let skill = skills.get(self.agent).unwrap();
 
-        if online {
-            Rating::new(skill.online, r.beta, r.drift)
-        } else if forward {
+        if forward {
             Rating::new(skill.forward, r.beta, r.drift)
         } else {
             Rating::new(skill.posterior() / self.likelihood, r.beta, r.drift)
@@ -107,7 +102,6 @@ impl Event {
 
     pub(crate) fn within_priors<T: Time, D: Drift<T>>(
         &self,
-        online: bool,
         forward: bool,
         skills: &SkillStore,
         agents: &CompetitorStore<T, D>,
@@ -117,7 +111,7 @@ impl Event {
             .map(|team| {
                 team.items
                     .iter()
-                    .map(|item| item.within_prior(online, forward, skills, agents))
+                    .map(|item| item.within_prior(forward, skills, agents))
                     .collect::<Vec<_>>()
             })
             .collect::<Vec<_>>()
@@ -136,7 +130,7 @@ impl Event {
         convergence: crate::ConvergenceOptions,
         arena: &mut ScratchArena,
     ) -> EventUpdate {
-        let teams = self.within_priors(false, false, skills, agents);
+        let teams = self.within_priors(false, skills, agents);
         let result = self.outputs();
         let g = match self.kind {
             EventKind::Ranked => {
@@ -372,7 +366,7 @@ impl<T: Time> TimeSlice<T> {
         if from > 0 || self.color_groups.is_empty() {
             // Initial pass (add_events) or no color groups yet: simple sequential sweep.
             for event in self.events.iter_mut().skip(from) {
-                let teams = event.within_priors(false, false, &self.skills, agents);
+                let teams = event.within_priors(false, &self.skills, agents);
                 let result = event.outputs();
 
                 let g = match event.kind {
@@ -582,7 +576,6 @@ impl<T: Time> TimeSlice<T> {
 
     pub(crate) fn log_evidence<D: Drift<T>>(
         &self,
-        online: bool,
         targets: &[Index],
         forward: bool,
         agents: &CompetitorStore<T, D>,
@@ -594,7 +587,7 @@ impl<T: Time> TimeSlice<T> {
         let mut arena = ScratchArena::new();
 
         let run_event = |event: &Event, arena: &mut ScratchArena| -> f64 {
-            let teams = event.within_priors(online, forward, &self.skills, agents);
+            let teams = event.within_priors(forward, &self.skills, agents);
             let result = event.outputs();
             match event.kind {
                 EventKind::Ranked => {
@@ -623,7 +616,7 @@ impl<T: Time> TimeSlice<T> {
         };
 
         if targets.is_empty() {
-            if online || forward {
+            if forward {
                 self.events
                     .iter()
                     .map(|event| run_event(event, &mut arena))
@@ -631,7 +624,7 @@ impl<T: Time> TimeSlice<T> {
             } else {
                 self.events.iter().map(|event| event.log_evidence).sum()
             }
-        } else if online || forward {
+        } else if forward {
             self.events
                 .iter()
                 .filter(|event| {