nika-event 0.61.0

Event Module - Event Sourcing for workflow execution

Provides full audit trail with replay capability. Key types:

• Event: Envelope with id + timestamp + kind
• EventKind: 58 variants across 15 categories
• EventLog: Thread-safe, append-only log
• EventEmitter: Trait for dependency injection
• NoopEmitter: Zero-cost no-op for testing
• TraceWriter: NDJSON file writer for debugging
• AgentTurnMetadata: Agent turn response metadata