feat(factor): introduce VarId and VarStore

Foundation types for the T1 factor graph machinery. VarStore is a
flat Vec<Gaussian> indexed by VarId; variables are allocated by
alloc() and the store can be cleared between games to reuse capacity.

Part of T1 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/lib.rs

@@ -13,6 +13,7 @@ pub(crate) mod arena;
 pub mod batch;
 pub mod drift;
 mod error;
+pub(crate) mod factor;
 mod game;
 pub mod gaussian;
 mod history;