Handle malformed events during JetStream replay with proper error reporting

Add ReplayError and ReplayResult types to capture information about
malformed events encountered during replay. This allows callers to
inspect and handle corrupted data rather than having it silently skipped.

Key changes:
- Add ReplayError type with sequence number, raw data, and underlying error
- Add ReplayResult type containing both successfully parsed events and errors
- Add EventStoreWithErrors interface for stores that can report replay errors
- Implement GetEventsWithErrors on JetStreamEventStore
- Update GetEvents to maintain backward compatibility (still skips malformed)
- Add comprehensive unit tests for the new types

This addresses the issue of silent data loss during event-sourced replay
by giving callers visibility into data quality issues.

Closes #39

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

event.go

@@ -28,6 +28,39 @@ func (e *VersionConflictError) Unwrap() error {
 	return ErrVersionConflict
 }
 
+// ReplayError captures information about a malformed event encountered during replay.
+// This allows callers to inspect and handle corrupted data without losing context.
+type ReplayError struct {
+	// SequenceNumber is the sequence number of the message in the stream (if available)
+	SequenceNumber uint64
+	// RawData is the raw bytes that could not be unmarshaled
+	RawData []byte
+	// Err is the underlying unmarshal error
+	Err error
+}
+
+func (e *ReplayError) Error() string {
+	return fmt.Sprintf("failed to unmarshal event at sequence %d: %v", e.SequenceNumber, e.Err)
+}
+
+func (e *ReplayError) Unwrap() error {
+	return e.Err
+}
+
+// ReplayResult contains the results of replaying events, including any errors encountered.
+// This allows callers to decide how to handle malformed events rather than silently skipping them.
+type ReplayResult struct {
+	// Events contains the successfully unmarshaled events
+	Events []*Event
+	// Errors contains information about any malformed events encountered
+	Errors []ReplayError
+}
+
+// HasErrors returns true if any malformed events were encountered during replay
+func (r *ReplayResult) HasErrors() bool {
+	return len(r.Errors) > 0
+}
+
 // Event represents a domain event in the system
 type Event struct {
 	ID        string                 `json:"id"`
@@ -174,6 +207,18 @@ type EventStore interface {
 	GetLatestVersion(actorID string) (int64, error)
 }
 
+// EventStoreWithErrors extends EventStore with methods that report malformed events.
+// Stores that may encounter corrupted data during replay (e.g., JetStream) should
+// implement this interface to give callers visibility into data quality issues.
+type EventStoreWithErrors interface {
+	EventStore
+
+	// GetEventsWithErrors retrieves events for an actor and reports any malformed
+	// events encountered. This method allows callers to decide how to handle
+	// corrupted data rather than silently skipping it.
+	GetEventsWithErrors(actorID string, fromVersion int64) (*ReplayResult, error)
+}
+
 // SnapshotStore extends EventStore with snapshot capabilities
 type SnapshotStore interface {
 	EventStore