Tighten evidence context source excerpts

internal/framework/evidencecontext/codec.go

@@ -167,6 +167,7 @@ func canonicalizeOwned(value Document) (Document, error) {
 		return nil, fmt.Errorf("document must be a JSON array")
 	}
 	seenUnitIDs := make(map[int]struct{}, len(value))
+	sourceID := ""
 	for unitIndex := range value {
 		unit := value[unitIndex]
 		if unit.ID <= 0 || strings.TrimSpace(unit.Kind) == "" || strings.TrimSpace(unit.Text) == "" {
@@ -175,6 +176,11 @@ func canonicalizeOwned(value Document) (Document, error) {
 		if err := validateUnitRef(unit, unitIndex); err != nil {
 			return nil, err
 		}
+		if sourceID == "" {
+			sourceID = unit.Ref.SourceID
+		} else if unit.Ref.SourceID != sourceID {
+			return nil, fmt.Errorf("units[%d].ref.source_id must match units[0].ref.source_id", unitIndex)
+		}
 		if _, exists := seenUnitIDs[unit.ID]; exists {
 			return nil, fmt.Errorf("units contains duplicate unit id %d", unit.ID)
 		}