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.

benches/batch.rs

@@ -1,9 +1,7 @@
-use std::collections::HashMap;
-
 use criterion::{Criterion, criterion_group, criterion_main};
 use trueskill_tt::{
     BETA, GAMMA, IndexMap, MU, P_DRAW, SIGMA, agent::Agent, batch::Batch, drift::ConstantDrift,
-    gaussian::Gaussian, player::Player,
+    gaussian::Gaussian, player::Player, storage::AgentStore,
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
@@ -13,33 +11,17 @@ fn criterion_benchmark(criterion: &mut Criterion) {
     let b = index.get_or_create("b");
     let c = index.get_or_create("c");
 
-    let agents = {
-        let mut map = HashMap::new();
+    let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
 
-        map.insert(
-            a,
+    for agent in [a, b, c] {
+        agents.insert(
+            agent,
             Agent {
                 player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
                 ..Default::default()
             },
         );
-        map.insert(
-            b,
-            Agent {
-                player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
-                ..Default::default()
-            },
-        );
-        map.insert(
-            c,
-            Agent {
-                player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
-                ..Default::default()
-            },
-        );
-
-        map
-    };
+    }
 
     let mut composition = Vec::new();
     let mut results = Vec::new();