Add comprehensive unit tests for InMemoryEventStore

- Test SaveEvent persists events correctly (single, multiple, multi-actor)
- Test GetEvents retrieves events in insertion order
- Test GetEvents with fromVersion filtering
- Test GetLatestVersion returns correct version
- Test behavior with non-existent actor IDs (returns empty/zero)
- Test concurrent access safety with race detector
- Add mutex protection to InMemoryEventStore for thread safety

Closes #3

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

store/memory.go

@@ -1,11 +1,14 @@
 package store
 
 import (
+	"sync"
+
 	"git.flowmade.one/flowmade-one/aether"
 )
 
 // InMemoryEventStore provides a simple in-memory event store for testing
 type InMemoryEventStore struct {
+	mu     sync.RWMutex
 	events map[string][]*aether.Event // actorID -> events
 }
 
@@ -18,6 +21,9 @@ func NewInMemoryEventStore() *InMemoryEventStore {
 
 // SaveEvent saves an event to the in-memory store
 func (es *InMemoryEventStore) SaveEvent(event *aether.Event) error {
+	es.mu.Lock()
+	defer es.mu.Unlock()
+
 	if _, exists := es.events[event.ActorID]; !exists {
 		es.events[event.ActorID] = make([]*aether.Event, 0)
 	}
@@ -27,6 +33,9 @@ func (es *InMemoryEventStore) SaveEvent(event *aether.Event) error {
 
 // GetEvents retrieves events for an actor from a specific version
 func (es *InMemoryEventStore) GetEvents(actorID string, fromVersion int64) ([]*aether.Event, error) {
+	es.mu.RLock()
+	defer es.mu.RUnlock()
+
 	events, exists := es.events[actorID]
 	if !exists {
 		return []*aether.Event{}, nil
@@ -44,6 +53,9 @@ func (es *InMemoryEventStore) GetEvents(actorID string, fromVersion int64) ([]*a
 
 // GetLatestVersion returns the latest version for an actor
 func (es *InMemoryEventStore) GetLatestVersion(actorID string) (int64, error) {
+	es.mu.RLock()
+	defer es.mu.RUnlock()
+
 	events, exists := es.events[actorID]
 	if !exists || len(events) == 0 {
 		return 0, nil