docs: Verify and document append-only immutability guarantees

Document that EventStore interface has no Update/Delete methods, enforcing
append-only semantics by design. Events are immutable once persisted.

Changes:
- Update EventStore interface documentation in event.go to explicitly state
  immutability guarantee and explain why Update/Delete methods are absent
- Add detailed retention policy documentation to JetStreamConfig showing
  how MaxAge limits enforce automatic expiration without manual deletion
- Document JetStreamEventStore's immutability guarantee with storage-level
  explanation of file-based storage and limits-based retention
- Add comprehensive immutability tests verifying:
  - Events cannot be modified after persistence
  - No Update or Delete methods exist on EventStore interface
  - Versions are monotonically increasing
  - Events cannot be deleted through the API
- Update README with detailed immutability section explaining:
  - Interface-level append-only guarantee
  - Storage-level immutability through JetStream configuration
  - Audit trail reliability
  - Pattern for handling corrections (append new event)

Closes #60

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

store/jetstream.go

@@ -20,7 +20,14 @@ const (
 
 // JetStreamConfig holds configuration options for JetStreamEventStore
 type JetStreamConfig struct {
-	// StreamRetention is how long to keep events (default: 1 year)
+	// StreamRetention is how long to keep events (default: 1 year).
+	// JetStream enforces this retention policy at the storage level using a limits-based policy:
+	// - MaxAge: Events older than this duration are automatically deleted
+	// - Storage is file-based (nats.FileStorage) for durability
+	// - Once the retention period expires, events are permanently removed from the stream
+	// This ensures that old events do not consume storage indefinitely.
+	// To keep events indefinitely, set StreamRetention to a very large value or configure
+	// a custom retention policy in the JetStream stream configuration.
 	StreamRetention time.Duration
 	// ReplicaCount is the number of replicas for high availability (default: 1)
 	ReplicaCount int
@@ -42,6 +49,21 @@ func DefaultJetStreamConfig() JetStreamConfig {
 // JetStreamEventStore implements EventStore using NATS JetStream for persistence.
 // It also implements EventStoreWithErrors to report malformed events during replay.
 //
+// ## Immutability Guarantee
+//
+// JetStreamEventStore is append-only. Events are stored in a JetStream stream that
+// is configured with file-based storage (nats.FileStorage) and a retention policy
+// (nats.LimitsPolicy). The configured MaxAge retention policy ensures that old events
+// eventually expire, but during their lifetime, events are never modified or deleted
+// through the EventStore API. Once an event is published to the stream:
+// - It cannot be updated
+// - It cannot be deleted before expiration
+// - It can only be read
+//
+// This architectural guarantee, combined with the EventStore interface providing
+// no Update or Delete methods, ensures events are immutable and suitable as an
+// audit trail.
+//
 // ## Version Cache Invalidation Strategy
 //
 // JetStreamEventStore maintains an in-memory cache of actor versions for optimistic
@@ -72,12 +94,6 @@ type JetStreamEventStore struct {
 
 
 
-
-
-
-
-
-
 // NewJetStreamEventStore creates a new JetStream-based event store with default configuration
 func NewJetStreamEventStore(natsConn *nats.Conn, streamName string) (*JetStreamEventStore, error) {
 	return NewJetStreamEventStoreWithConfig(natsConn, streamName, DefaultJetStreamConfig())