Sprint 2: update runtime __init__ with checkpoint + state exports

Sprint 2: checkpoint.py — Checkpointer protocol + InMemory/JSONL/SQLite implementations

Sprint 2: runtime/state.py — typed RunState for durable execution

Sprint 1: EventBus — async pub/sub with backpressure, replay, lane isolation

Sprint 1: canonical event schema (PAEvent, EventKind, Visibility)

Sprint 1: runtime package init