refactor(batch): replace HashMap<Index, Skill> with dense SkillStore

SkillStore is a Vec<Skill>-backed dense store with a parallel present
mask, indexed directly by Index.0. Eliminates per-iteration hashing
in the within-slice convergence loop; O(1) array lookup replaces O(1)
amortised hash lookup with better cache behaviour.

Iteration order is now ascending-by-Index (was arbitrary for HashMap);
EP fixed point is order-independent so posteriors are unchanged.

Part of T0 engine redesign.

src/lib.rs

@@ -18,6 +18,7 @@ mod history;
 mod matrix;
 mod message;
 pub mod player;
+pub(crate) mod storage;
 
 pub use drift::{ConstantDrift, Drift};
 pub use error::InferenceError;