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>

store/memory.go

@@ -0,0 +1,60 @@
+package store
+
+import (
+	"git.flowmade.one/flowmade-one/aether"
+)
+
+// InMemoryEventStore provides a simple in-memory event store for testing
+type InMemoryEventStore struct {
+	events map[string][]*aether.Event // actorID -> events
+}
+
+// NewInMemoryEventStore creates a new in-memory event store
+func NewInMemoryEventStore() *InMemoryEventStore {
+	return &InMemoryEventStore{
+		events: make(map[string][]*aether.Event),
+	}
+}
+
+// SaveEvent saves an event to the in-memory store
+func (es *InMemoryEventStore) SaveEvent(event *aether.Event) error {
+	if _, exists := es.events[event.ActorID]; !exists {
+		es.events[event.ActorID] = make([]*aether.Event, 0)
+	}
+	es.events[event.ActorID] = append(es.events[event.ActorID], event)
+	return nil
+}
+
+// GetEvents retrieves events for an actor from a specific version
+func (es *InMemoryEventStore) GetEvents(actorID string, fromVersion int64) ([]*aether.Event, error) {
+	events, exists := es.events[actorID]
+	if !exists {
+		return []*aether.Event{}, nil
+	}
+
+	var filteredEvents []*aether.Event
+	for _, event := range events {
+		if event.Version >= fromVersion {
+			filteredEvents = append(filteredEvents, event)
+		}
+	}
+
+	return filteredEvents, nil
+}
+
+// GetLatestVersion returns the latest version for an actor
+func (es *InMemoryEventStore) GetLatestVersion(actorID string) (int64, error) {
+	events, exists := es.events[actorID]
+	if !exists || len(events) == 0 {
+		return 0, nil
+	}
+
+	latestVersion := int64(0)
+	for _, event := range events {
+		if event.Version > latestVersion {
+			latestVersion = event.Version
+		}
+	}
+
+	return latestVersion, nil
+}