sequence.go
func (s *FileSequence) frameInt(frame int) string {
	var zframe string
	if s.frameSet != nil {
		zframe = zfillInt(frame, s.zfill)
	}
	var buf strings.Builder
	buf.WriteString(s.dir)
	buf.WriteString(s.basename)
	buf.WriteString(zframe)
	buf.WriteString(s.ext)
	return buf.String()
}
func (s *FileSequence) frameInt(frame int) string {
	var zframe string
	if s.frameSet != nil {
		zframe = zfillInt(frame, s.zfill)
	}
	var buf strings.Builder
	buf.WriteString(s.dir)
	buf.WriteString(s.basename)
	buf.WriteString(zframe)
	buf.WriteString(s.ext)
	return buf.String()
}
// Internal fast path for Frame() when we know we have an int frame