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>

event.go

@@ -184,6 +184,17 @@ type ActorSnapshot struct {
 
 // EventStore defines the interface for event persistence.
 //
+// # Immutability Guarantee
+//
+// EventStore is append-only. Once an event is persisted via SaveEvent, it is never
+// modified or deleted. The interface intentionally provides no Update or Delete methods.
+// This ensures:
+//   - Events serve as an immutable audit trail
+//   - State can be safely derived by replaying events
+//   - Concurrent reads are always safe (events never change)
+//
+// To correct a mistake, append a new event that expresses the correction.
+//
 // # Version Semantics
 //
 // Events for an actor must have monotonically increasing versions. When SaveEvent
@@ -204,10 +215,13 @@ type EventStore interface {
 	// SaveEvent persists an event to the store. The event's Version must be
 	// strictly greater than the current latest version for the actor.
 	// Returns VersionConflictError if version <= current latest version.
+	// Once saved, the event is immutable and can never be modified or deleted.
 	SaveEvent(event *Event) error
 
 	// GetEvents retrieves events for an actor from a specific version (inclusive).
 	// Returns an empty slice if no events exist for the actor.
+	// The returned events are guaranteed to be immutable - they will never be
+	// modified or deleted from the store.
 	GetEvents(actorID string, fromVersion int64) ([]*Event, error)
 
 	// GetLatestVersion returns the latest version for an actor.