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/lib.rs

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