areev-run-core — the pure scheduler of areev run

Sans-IO by construction (governed-agents proposal §4): this crate cannot observe the world. It exports [step] — a pure function from (plan, env, state, events) to (commands, state) — and the vocabulary around it. The driver crate (areev-run) owns the store, the clock, the executors, and the journal; it materializes journal lookups into [EventIn]s and performs [Command]s. Replay IS this function: feed the journaled events back in and assert the same commands come out.

Purity is enforced mechanically, not aspirationally: the dependency tree carries no clock/rand/net crate (CI checks cargo tree), and this crate's clippy.toml disallows SystemTime::now/Instant::now. Scheduler state uses Vec/BTreeMap exclusively — hash-iteration order is the classic silent replay-divergence source and has no business here.