Initial aether repository structure

Distributed actor system with event sourcing for Go:
- event.go - Event, ActorSnapshot, EventStore interface
- eventbus.go - EventBus, EventBroadcaster for pub/sub
- nats_eventbus.go - NATS-backed cross-node event broadcasting
- store/ - InMemoryEventStore (testing), JetStreamEventStore (production)
- cluster/ - Node discovery, leader election, shard distribution
- model/ - EventStorming model types

Extracted from arcadia as open-source infrastructure component.

Co-Authored-By: Claude <noreply@anthropic.com>

event.go

@@ -0,0 +1,38 @@
+package aether
+
+import (
+	"time"
+)
+
+// Event represents a domain event in the system
+type Event struct {
+	ID        string                 `json:"id"`
+	EventType string                 `json:"eventType"`
+	ActorID   string                 `json:"actorId"`
+	CommandID string                 `json:"commandId,omitempty"` // Correlation ID for command that triggered this event
+	Version   int64                  `json:"version"`
+	Data      map[string]interface{} `json:"data"`
+	Timestamp time.Time              `json:"timestamp"`
+}
+
+// ActorSnapshot represents a point-in-time state snapshot
+type ActorSnapshot struct {
+	ActorID   string                 `json:"actorId"`
+	Version   int64                  `json:"version"`
+	State     map[string]interface{} `json:"state"`
+	Timestamp time.Time              `json:"timestamp"`
+}
+
+// EventStore defines the interface for event persistence
+type EventStore interface {
+	SaveEvent(event *Event) error
+	GetEvents(actorID string, fromVersion int64) ([]*Event, error)
+	GetLatestVersion(actorID string) (int64, error)
+}
+
+// SnapshotStore extends EventStore with snapshot capabilities
+type SnapshotStore interface {
+	EventStore
+	GetLatestSnapshot(actorID string) (*ActorSnapshot, error)
+	SaveSnapshot(snapshot *ActorSnapshot) error
+}