feat: add History::posterior_of for a linear combination of competitors

#46: every accessor returns a per-competitor marginal, and almost
nothing a consumer publishes is one competitor. Combining marginals
assumes independence, and competitors are correlated through every event
they share.

`posterior_of(&[(a, 1.0), (b, -1.0)])` returns the posterior of that
combination with the correlation intact. Validated against the exact
linear-Gaussian posterior on both a tree and a loopy fixture, for
differences and for single competitors: agreement to 1e-9 relative in
every case.

The investigation that preceded this is why it is not a covariance
accessor. Marginals from loopy message passing are about half the true
width, and ignoring correlation overstates a difference — the two errors
partially cancel, leaving 1.327x rather than 2.646x. Bolting true
correlations onto the existing marginals would have given 0.765 against
a true 1.524, which is overconfident: the direction the reporter
specifically called unsafe. Rebuilding the joint from the factor
structure fixes both at once, and a single-competitor query now returns
the exact marginal rather than the narrow one.

The precision matrix depends only on structure — who played whom, with
what weights and what noise — not on the observed outcomes, and the
means were already exact. So only the second moment is reconstructed.

Known limits, all deliberate and documented on the method:

- Latest slice only. A functional spanning times, such as "current
  versus career", needs the time-expanded joint and is not covered.
- Scored events only. A ranked outcome's truncation is EP-approximated
  and its converged factors are not retained after inference, so ranked
  slices return `JointUnavailable` rather than a plausible wrong number.
- Dense Cholesky, O(n^3) per query in the slice's competitor count:
  38.8us at 50, 5.66ms at 400, 49.1ms at 800. Fine for the sizes this
  serves today; caching the factorization per slice would make repeat
  queries O(n^2), and sparsity is the next step after that.

Refs #46, #47, #48

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

src/error.rs

@@ -102,6 +102,8 @@ pub enum InferenceError {
     },
     /// A prediction was given a team with no members.
     EmptyTeam { team: usize },
+    /// A joint posterior was requested where one cannot be formed exactly.
+    JointUnavailable { reason: &'static str },
     /// Fewer than two teams were supplied to a prediction.
     NotEnoughTeams { got: usize },
     /// The full outcome distribution was requested for too many teams.
@@ -168,6 +170,9 @@ impl fmt::Display for InferenceError {
             Self::EmptyTeam { team } => {
                 write!(f, "team {team} has no members")
             }
+            Self::JointUnavailable { reason } => {
+                write!(f, "no exact joint posterior is available: {reason}")
+            }
             Self::NotEnoughTeams { got } => {
                 write!(f, "prediction needs at least 2 teams, got {got}")
             }