Centralize segment reference fallback in model

internal/model/model.go

@@ -1,5 +1,7 @@
 package model
 
+import "fmt"
+
 // RawTranscript is a loaded input document before canonical normalization.
 type RawTranscript struct {
 	Source   string       `json:"source"`
@@ -61,6 +63,17 @@ type Segment struct {
 	OverlapGroupID     int      `json:"overlap_group_id,omitempty"`
 }
 
+// SegmentReference returns the best available external reference for a segment.
+func SegmentReference(segment Segment) string {
+	if segment.Source != "" && segment.SourceSegmentIndex != nil {
+		return fmt.Sprintf("%s#%d", segment.Source, *segment.SourceSegmentIndex)
+	}
+	if segment.SourceRef != "" {
+		return segment.SourceRef
+	}
+	return ""
+}
+
 // Word preserves optional word-level timing data.
 type Word struct {
 	Text    string  `json:"text"`