Game::likelihoods now uses VarStore (for diff vars) and TruncFactor
(for EP truncation + evidence caching) instead of TeamMessage and
DiffMessage. The EP loop structure is preserved exactly; VarId-keyed
diff vars live in the arena's VarStore (capacity reused per batch).
ScratchArena loses teams/diffs/ties/margins; gains VarStore and
sort_buf (sort_perm allocation eliminated). message.rs deleted.
Public API of Game (new, posteriors, likelihoods, evidence) unchanged.
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.
