Add event versioning validation

- Add ErrVersionConflict error type and VersionConflictError for detailed
  conflict information
- Implement version validation in InMemoryEventStore.SaveEvent that rejects
  events with version <= current latest version
- Implement version validation in JetStreamEventStore.SaveEvent with version
  caching for performance
- Add comprehensive tests for version conflict detection including concurrent
  writes to same actor
- Document versioning semantics in EventStore interface and CLAUDE.md

This ensures events have monotonically increasing versions per actor and
provides clear error messages for version conflicts, enabling optimistic
concurrency control patterns.

Closes #6

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

store/memory.go

@@ -1,7 +1,6 @@
 package store
 
 import (
-	"fmt"
 	"sync"
 
 	"git.flowmade.one/flowmade-one/aether"
@@ -22,11 +21,32 @@ func NewInMemoryEventStore() *InMemoryEventStore {
 	}
 }
 
-// SaveEvent saves an event to the in-memory store
+// SaveEvent saves an event to the in-memory store.
+// Returns VersionConflictError if the event's version is not strictly greater
+// than the current latest version for the actor.
 func (es *InMemoryEventStore) SaveEvent(event *aether.Event) error {
 	es.mu.Lock()
 	defer es.mu.Unlock()
 
+	// Get current latest version for this actor
+	currentVersion := int64(0)
+	if events, exists := es.events[event.ActorID]; exists {
+		for _, e := range events {
+			if e.Version > currentVersion {
+				currentVersion = e.Version
+			}
+		}
+	}
+
+	// Validate version is strictly greater than current
+	if event.Version <= currentVersion {
+		return &aether.VersionConflictError{
+			ActorID:          event.ActorID,
+			AttemptedVersion: event.Version,
+			CurrentVersion:   currentVersion,
+		}
+	}
+
 	if _, exists := es.events[event.ActorID]; !exists {
 		es.events[event.ActorID] = make([]*aether.Event, 0)
 	}
@@ -77,7 +97,7 @@ func (es *InMemoryEventStore) GetLatestVersion(actorID string) (int64, error) {
 // SaveSnapshot saves a snapshot to the in-memory store
 func (es *InMemoryEventStore) SaveSnapshot(snapshot *aether.ActorSnapshot) error {
 	if snapshot == nil {
-		return fmt.Errorf("snapshot cannot be nil")
+		return &snapshotNilError{}
 	}
 
 	es.mu.Lock()
@@ -109,3 +129,10 @@ func (es *InMemoryEventStore) GetLatestSnapshot(actorID string) (*aether.ActorSn
 
 	return latest, nil
 }
+
+// snapshotNilError is returned when attempting to save a nil snapshot
+type snapshotNilError struct{}
+
+func (e *snapshotNilError) Error() string {
+	return "snapshot cannot be nil"
+}