Events form a Crystal class hierarchy so subscribers pattern-match
exhaustively. EventBus delivers to subscribers via Crystal channels with
per-subscriber overflow policy (Block for audit, Drop for best-effort).
AuditSubscriber listens for all rotation/policy/drift/alert events and
serializes them into the hash-chained audit log via AuditLog.append.
Engine wires Persistence + AuditLog + AuditSubscriber + EventBus together
so callers just .start/.stop. 7 unit specs verify boot, fanout to N
subscribers, drop-on-overflow semantics, and end-to-end event -> audit row.
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