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/storage/mod.rs

@@ -0,0 +1,3 @@
+mod skill_store;
+
+pub(crate) use skill_store::SkillStore;