perf(game): eliminate per-event allocations via ScratchArena

Game::likelihoods previously allocated four Vecs (teams, diffs, ties,
margins) on every call. Batch now owns one ScratchArena reused across
all Game::new calls in the iteration loop; likelihoods() clears and
extends the arena buffers instead of allocating fresh.

For log_evidence (called infrequently), a local ScratchArena is created
per invocation so the method signature stays &self.

Also: add #[derive(Debug)] to TeamMessage and DiffMessage (required by
ScratchArena's own Debug derive).

Part of T0 engine redesign.

src/history.rs

@@ -436,7 +436,10 @@ mod tests {
     use approx::assert_ulps_eq;
 
     use super::*;
-    use crate::{ConstantDrift, EPSILON, Game, Gaussian, ITERATIONS, IndexMap, P_DRAW, Player};
+    use crate::{
+        ConstantDrift, EPSILON, Game, Gaussian, ITERATIONS, IndexMap, P_DRAW, Player,
+        arena::ScratchArena,
+    };
 
     #[test]
     fn test_init() {
@@ -500,6 +503,7 @@ mod tests {
             &[0.0, 1.0],
             &w,
             P_DRAW,
+            &mut ScratchArena::new(),
         )
         .posteriors();
         let expected = p[0][0];