refactor(history): replace HashMap<Index, Agent<D>> with dense AgentStore<D>

AgentStore<D> is a Vec<Option<Agent<D>>>-backed store indexed directly
by Index.0, eliminating per-iteration hashing in the cross-history
forward/backward sweep. Implements Index<Index>/IndexMut<Index> for
ergonomic agent access.

AgentStore is public (so benches/batch.rs can use it). SkillStore
remains pub(crate) since Skill is pub(crate) in batch.rs.

HashMap<Index, _> is now only used for the posteriors() return value
(temporary; will be replaced in T2 with a proper typed return) and
for the add_events_with_prior(priors: HashMap<Index, Player<D>>) API
(also T2 target).

Part of T0 engine redesign.

src/storage/mod.rs

@@ -1,3 +1,5 @@
+mod agent_store;
 mod skill_store;
 
+pub use agent_store::AgentStore;
 pub(crate) use skill_store::SkillStore;