The former name collided with the popular indexmap crate. KeyTable
lives in its own module. Public API unchanged beyond the rename.
Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.
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.
