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/error.rs

@@ -10,6 +10,8 @@ pub enum InferenceError {
     },
     /// A probability value is outside `[0, 1]`.
     InvalidProbability { value: f64 },
+    /// A scalar parameter is outside its valid range.
+    InvalidParameter { name: &'static str, value: f64 },
     /// Convergence exceeded `max_iter` without falling below `epsilon`.
     ConvergenceFailed {
         last_step: (f64, f64),
@@ -32,6 +34,9 @@ impl fmt::Display for InferenceError {
             Self::InvalidProbability { value } => {
                 write!(f, "probability must be in [0, 1]; got {value}")
             }
+            Self::InvalidParameter { name, value } => {
+                write!(f, "{name} is invalid: {value}")
+            }
             Self::ConvergenceFailed {
                 last_step,
                 iterations,