T4 (MarginFactor): scored outcomes via Gaussian-margin EP evidence

Adds soft Gaussian-observation evidence on the per-pair diff variable,
enabling continuous score margins as a richer alternative to ranks.

Public API:
- `Outcome::Scored([scores])` (non-breaking enum extension under
  `#[non_exhaustive]`).
- `Game::scored(teams, outcome, options)` constructor parallel to
  `Game::ranked`.
- `EventBuilder::scores([...])` fluent helper.
- `HistoryBuilder::score_sigma(σ)` knob (default 1.0, validated > 0).
- `GameOptions::score_sigma`.
- `EventKind` re-exported from `lib.rs` (annotated `#[non_exhaustive]`).
- New `InferenceError::InvalidParameter { name, value }` variant.

Internals:
- `MarginFactor` (`factor/margin.rs`): Gaussian observation factor that
  closes in one EP step; cavity-cached log-evidence mirrors `TruncFactor`.
- `BuiltinFactor::Margin` dispatch arm.
- `DiffFactor` enum in `game.rs` lets `Game::likelihoods` and the new
  `likelihoods_scored` share the per-pair link abstraction.
- Per-event `EventKind { Ranked, Scored { score_sigma } }` routed through
  `TimeSlice::add_events`, `iteration_direct`, and `log_evidence`.

Tests: 88 lib + 27 integration (4 new in `tests/scored.rs`); existing
goldens byte-identical.  Bench: `benches/scored.rs` baseline ~960µs for
60 events × 20-player pool with default convergence.

Plan: docs/superpowers/plans/2026-04-27-t4-margin-factor.md
Spec item marked Done.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/outcome.rs

@@ -1,8 +1,7 @@
 //! Outcome of a match.
 //!
-//! In T2, only `Ranked` is supported; `Scored` will be added together with
-//! `MarginFactor` in T4. The enum is `#[non_exhaustive]` so adding `Scored`
-//! is non-breaking for downstream `match` expressions.
+//! `Ranked(ranks)` for ordinal results; `Scored(scores)` for continuous
+//! per-team scores (engages `MarginFactor` in the engine).
 
 use smallvec::SmallVec;
 
@@ -10,14 +9,19 @@ use smallvec::SmallVec;
 ///
 /// `Ranked(ranks)`: lower rank = better. Equal ranks mean a tie between those
 /// teams. `ranks.len()` must equal the number of teams in the event.
+///
+/// `Scored(scores)`: higher score = better. Adjacent (sorted) pairs feed
+/// observed margins to `MarginFactor`. `scores.len()` must equal the number
+/// of teams in the event.
 #[derive(Clone, Debug, PartialEq)]
 #[non_exhaustive]
 pub enum Outcome {
     Ranked(SmallVec<[u32; 4]>),
+    Scored(SmallVec<[f64; 4]>),
 }
 
 impl Outcome {
-    /// `N`-team outcome where team `winner` won and everyone else tied for last.
+    /// `n`-team outcome where team `winner` won and everyone else tied for last.
     ///
     /// Panics if `winner >= n`.
     pub fn winner(winner: u32, n: u32) -> Self {
@@ -36,16 +40,29 @@ impl Outcome {
         Self::Ranked(ranks.into_iter().collect())
     }
 
+    /// Explicit per-team continuous scores; higher = better.
+    pub fn scores<I: IntoIterator<Item = f64>>(scores: I) -> Self {
+        Self::Scored(scores.into_iter().collect())
+    }
+
     pub fn team_count(&self) -> usize {
         match self {
             Self::Ranked(r) => r.len(),
+            Self::Scored(s) => s.len(),
         }
     }
 
-    #[allow(dead_code)]
-    pub(crate) fn as_ranks(&self) -> &[u32] {
+    pub(crate) fn as_ranks(&self) -> Option<&[u32]> {
         match self {
-            Self::Ranked(r) => r,
+            Self::Ranked(r) => Some(r),
+            Self::Scored(_) => None,
+        }
+    }
+
+    pub(crate) fn as_scores(&self) -> Option<&[f64]> {
+        match self {
+            Self::Scored(s) => Some(s),
+            Self::Ranked(_) => None,
         }
     }
 }
@@ -57,26 +74,26 @@ mod tests {
     #[test]
     fn winner_two_teams() {
         let o = Outcome::winner(0, 2);
-        assert_eq!(o.as_ranks(), &[0u32, 1]);
+        assert_eq!(o.as_ranks(), Some(&[0u32, 1][..]));
         assert_eq!(o.team_count(), 2);
     }
 
     #[test]
     fn winner_three_teams_second_wins() {
         let o = Outcome::winner(1, 3);
-        assert_eq!(o.as_ranks(), &[1u32, 0, 1]);
+        assert_eq!(o.as_ranks(), Some(&[1u32, 0, 1][..]));
     }
 
     #[test]
     fn draw_three_teams() {
         let o = Outcome::draw(3);
-        assert_eq!(o.as_ranks(), &[0u32, 0, 0]);
+        assert_eq!(o.as_ranks(), Some(&[0u32, 0, 0][..]));
     }
 
     #[test]
     fn ranking_from_iter() {
         let o = Outcome::ranking([2, 0, 1]);
-        assert_eq!(o.as_ranks(), &[2u32, 0, 1]);
+        assert_eq!(o.as_ranks(), Some(&[2u32, 0, 1][..]));
     }
 
     #[test]
@@ -84,4 +101,25 @@ mod tests {
     fn winner_out_of_range_panics() {
         let _ = Outcome::winner(2, 2);
     }
+
+    #[test]
+    fn scored_two_teams() {
+        let o = Outcome::scores([10.0, 4.0]);
+        assert_eq!(o.team_count(), 2);
+        assert_eq!(o.as_scores(), Some(&[10.0, 4.0][..]));
+        assert_eq!(o.as_ranks(), None);
+    }
+
+    #[test]
+    fn scored_team_count_matches_input() {
+        let o = Outcome::scores([3.0, 1.0, 2.0, 0.0]);
+        assert_eq!(o.team_count(), 4);
+    }
+
+    #[test]
+    fn ranked_as_scores_returns_none() {
+        let o = Outcome::winner(0, 2);
+        assert!(o.as_scores().is_none());
+        assert!(o.as_ranks().is_some());
+    }
 }