Add comprehensive unit tests for SnapshotStore

Implement SaveSnapshot and GetLatestSnapshot methods on InMemoryEventStore
to satisfy the SnapshotStore interface, and add comprehensive tests covering:

- SaveSnapshot persists snapshots correctly
- GetLatestSnapshot returns most recent snapshot by version
- Behavior when no snapshot exists (returns nil, nil)
- Snapshot versioning is respected across actors
- Version ordering (higher version wins, not insertion order)
- Data integrity for complex, nested, and special character states
- Edge cases: zero version, large version, empty/nil state

Closes #4

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

store/memory.go

@@ -6,13 +6,15 @@ import (
 
 // InMemoryEventStore provides a simple in-memory event store for testing
 type InMemoryEventStore struct {
-	events map[string][]*aether.Event // actorID -> events
+	events    map[string][]*aether.Event         // actorID -> events
+	snapshots map[string][]*aether.ActorSnapshot // actorID -> snapshots (sorted by version)
 }
 
 // NewInMemoryEventStore creates a new in-memory event store
 func NewInMemoryEventStore() *InMemoryEventStore {
 	return &InMemoryEventStore{
-		events: make(map[string][]*aether.Event),
+		events:    make(map[string][]*aether.Event),
+		snapshots: make(map[string][]*aether.ActorSnapshot),
 	}
 }
 
@@ -58,3 +60,29 @@ func (es *InMemoryEventStore) GetLatestVersion(actorID string) (int64, error) {
 
 	return latestVersion, nil
 }
+
+// SaveSnapshot saves a snapshot to the in-memory store
+func (es *InMemoryEventStore) SaveSnapshot(snapshot *aether.ActorSnapshot) error {
+	if _, exists := es.snapshots[snapshot.ActorID]; !exists {
+		es.snapshots[snapshot.ActorID] = make([]*aether.ActorSnapshot, 0)
+	}
+	es.snapshots[snapshot.ActorID] = append(es.snapshots[snapshot.ActorID], snapshot)
+	return nil
+}
+
+// GetLatestSnapshot returns the most recent snapshot for an actor
+func (es *InMemoryEventStore) GetLatestSnapshot(actorID string) (*aether.ActorSnapshot, error) {
+	snapshots, exists := es.snapshots[actorID]
+	if !exists || len(snapshots) == 0 {
+		return nil, nil
+	}
+
+	var latest *aether.ActorSnapshot
+	for _, snapshot := range snapshots {
+		if latest == nil || snapshot.Version > latest.Version {
+			latest = snapshot
+		}
+	}
+
+	return latest, nil
+}