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>

CLAUDE.md

@@ -79,6 +79,49 @@ store.SaveEvent(event)
 events, _ := store.GetEvents("order-123", 0)
 ```
 
+### Event Versioning
+
+Events for each actor must have **monotonically increasing versions**. This ensures event stream integrity and enables optimistic concurrency control.
+
+#### Version Semantics
+
+- Each actor has an independent version sequence
+- Version must be strictly greater than the current latest version
+- For new actors (no events), the first event must have version > 0
+- Non-consecutive versions are allowed (gaps are permitted)
+
+#### Optimistic Concurrency Pattern
+
+```go
+// 1. Get current version
+currentVersion, _ := store.GetLatestVersion("order-123")
+
+// 2. Create event with next version
+event := &aether.Event{
+    ID:        uuid.New().String(),
+    EventType: "OrderUpdated",
+    ActorID:   "order-123",
+    Version:   currentVersion + 1,
+    Data:      map[string]interface{}{"status": "shipped"},
+    Timestamp: time.Now(),
+}
+
+// 3. Attempt to save
+err := store.SaveEvent(event)
+if errors.Is(err, aether.ErrVersionConflict) {
+    // Another writer won - reload and retry if appropriate
+    var versionErr *aether.VersionConflictError
+    errors.As(err, &versionErr)
+    log.Printf("Conflict: actor %s has version %d, attempted %d",
+        versionErr.ActorID, versionErr.CurrentVersion, versionErr.AttemptedVersion)
+}
+```
+
+#### Error Types
+
+- `ErrVersionConflict` - Sentinel error for version conflicts (use with `errors.Is`)
+- `VersionConflictError` - Detailed error with ActorID, CurrentVersion, and AttemptedVersion
+
 ### Namespace Isolation
 
 Namespaces provide logical boundaries for events and subscriptions:
@@ -111,6 +154,7 @@ if manager.IsLeader() {
 ## Key Patterns
 
 - **Events are immutable** - Never modify, only append
+- **Versions are monotonic** - Each event must have version > previous for same actor
 - **Snapshots for performance** - Periodically snapshot state to avoid full replay
 - **Namespaces for isolation** - Not multi-tenancy, just logical boundaries
 - **NATS for everything** - Events, pub/sub, clustering all use NATS