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/history.rs

@@ -1,7 +1,7 @@
 use std::{borrow::Borrow, collections::HashMap, hash::Hash, marker::PhantomData};
 
 use crate::{
-    BETA, GAMMA, Index, MU, N_INF, P_DRAW, SIGMA,
+    BETA, GAMMA, Index, MU, P_DRAW, SIGMA,
     competitor::{self, Competitor},
     convergence::{ConvergenceOptions, ConvergenceReport},
     drift::{ConstantDrift, Drift},
@@ -254,7 +254,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
         for j in (0..self.time_slices.len() - 1).rev() {
             for agent in self.time_slices[j + 1].skills.keys() {
                 self.agents.get_mut(agent).unwrap().message =
-                    self.time_slices[j + 1].backward_prior_out(&agent, &self.agents);
+                    Some(self.time_slices[j + 1].backward_prior_out(&agent, &self.agents));
             }
 
             let old = self.time_slices[j].posteriors();
@@ -273,7 +273,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
         for j in 1..self.time_slices.len() {
             for agent in self.time_slices[j - 1].skills.keys() {
                 self.agents.get_mut(agent).unwrap().message =
-                    self.time_slices[j - 1].forward_prior_out(&agent);
+                    Some(self.time_slices[j - 1].forward_prior_out(&agent));
             }
 
             let old = self.time_slices[j].posteriors();
@@ -723,7 +723,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                                 self.drift,
                             )
                         }),
-                        message: N_INF,
+                        message: None,
                         last_time: None,
                     },
                 );
@@ -761,7 +761,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                         let agent = self.agents.get_mut(*agent_idx).unwrap();
 
                         agent.last_time = Some(time_slice.time);
-                        agent.message = time_slice.forward_prior_out(agent_idx);
+                        agent.message = Some(time_slice.forward_prior_out(agent_idx));
                     }
                 }
 
@@ -794,7 +794,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                     let agent = self.agents.get_mut(agent_idx).unwrap();
 
                     agent.last_time = Some(t);
-                    agent.message = time_slice.forward_prior_out(&agent_idx);
+                    agent.message = Some(time_slice.forward_prior_out(&agent_idx));
                 }
 
                 k += 1;
@@ -810,7 +810,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                     let agent = self.agents.get_mut(agent_idx).unwrap();
 
                     agent.last_time = Some(t);
-                    agent.message = time_slice.forward_prior_out(&agent_idx);
+                    agent.message = Some(time_slice.forward_prior_out(&agent_idx));
                 }
 
                 k += 1;
@@ -834,7 +834,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                     let agent = self.agents.get_mut(*agent_idx).unwrap();
 
                     agent.last_time = Some(time_slice.time);
-                    agent.message = time_slice.forward_prior_out(agent_idx);
+                    agent.message = Some(time_slice.forward_prior_out(agent_idx));
                 }
             }