refactor!: make Competitor::message an Option, and compute_elapsed loud

Two of the five remaining items on #23.

`Competitor.message` was a `Gaussian` using the improper `N_INF` as an "unset"
sentinel, so `message != N_INF` meant "has a message" and every reader had to
know that convention. It is now `Option<Gaussian>`, which makes "no message
yet" and "a legitimately improper message" distinguishable at the type level
instead of by float comparison.

Worth noting what the change surfaced: switching the type turned every read
site into a compile error, and there were eight — two in the convergence sweep,
five in ingestion, one in new_backward_info. The last is the interesting one:
`skill.backward = agents[agent].message` needed `unwrap_or(N_INF)` rather than
an unwrap, because an absent message genuinely does mean the improper identity
there. A sentinel-based refactor would have had to find that by reading.

This is a breaking change: `message` is a public field. It rides the next
minor bump.

`compute_elapsed` clamped a negative elapsed to zero silently. Negative elapsed
means slices are being visited out of time order, which would otherwise make
drift *reduce* uncertainty. Release still clamps, so a bad timestamp degrades
to "no drift" rather than corrupting a posterior, but debug now trips — getting
there is a slice-ordering bug, not something callers can cause with ordinary
data.

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

src/competitor.rs

@@ -1,5 +1,4 @@
 use crate::{
-    N_INF,
     drift::{ConstantDrift, Drift},
     gaussian::Gaussian,
     rating::Rating,
@@ -13,7 +12,14 @@ use crate::{
 #[derive(Debug)]
 pub struct Competitor<T: Time = i64, D: Drift<T> = ConstantDrift> {
     pub rating: Rating<T, D>,
-    pub message: Gaussian,
+    /// The forward message carried from this competitor's last appearance, or
+    /// `None` before they have appeared anywhere.
+    ///
+    /// Previously an improper `N_INF` served as the unset sentinel, which made
+    /// "no message yet" indistinguishable from "a legitimately improper
+    /// message" at the type level and required every reader to know the
+    /// convention.
+    pub message: Option<Gaussian>,
     pub last_time: Option<T>,
 }
 
@@ -21,14 +27,16 @@ impl<T: Time, D: Drift<T>> Competitor<T, D> {
     /// Compute the message received at time `now`, with drift accumulated
     /// from `self.last_time` (if any) to `now`.
     pub(crate) fn receive(&self, now: &T) -> Gaussian {
-        if self.message != N_INF {
-            let elapsed_variance = match &self.last_time {
-                Some(last) => self.rating.drift.variance_delta(last, now),
-                None => 0.0,
-            };
-            self.message.forget(elapsed_variance)
-        } else {
-            self.rating.prior
+        match self.message {
+            Some(message) => {
+                let elapsed_variance = match &self.last_time {
+                    Some(last) => self.rating.drift.variance_delta(last, now),
+                    None => 0.0,
+                };
+
+                message.forget(elapsed_variance)
+            }
+            None => self.rating.prior,
         }
     }
 
@@ -37,11 +45,9 @@ impl<T: Time, D: Drift<T>> Competitor<T, D> {
     /// Used in convergence sweeps where the elapsed was cached at slice-construction time
     /// and should not be recomputed from `last_time` (which may have shifted).
     pub(crate) fn receive_for_elapsed(&self, elapsed: i64) -> Gaussian {
-        if self.message != N_INF {
-            self.message
-                .forget(self.rating.drift.variance_for_elapsed(elapsed))
-        } else {
-            self.rating.prior
+        match self.message {
+            Some(message) => message.forget(self.rating.drift.variance_for_elapsed(elapsed)),
+            None => self.rating.prior,
         }
     }
 }
@@ -50,7 +56,7 @@ impl Default for Competitor<i64, ConstantDrift> {
     fn default() -> Self {
         Self {
             rating: Rating::default(),
-            message: N_INF,
+            message: None,
             last_time: None,
         }
     }
@@ -63,7 +69,7 @@ where
     C: Iterator<Item = &'a mut Competitor<T, D>>,
 {
     for c in competitors {
-        c.message = N_INF;
+        c.message = None;
         if last_time {
             c.last_time = None;
         }