Files: a seek costs nothing; the backend reopens only when a read wants somewhere else

ServeContent seeks to the end for the size before it seeks to the range, which was
throwing away the stream Check had just opened.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/files/seek.go

@@ -26,7 +26,7 @@ func NewSeeker(ctx context.Context, fsys FS, path string, size int64) *Seeker {
 }
 
 // Check opens the file now, from the start, so a file that cannot be read is known before
-// any headers go out. A later seek elsewhere simply reopens.
+// any headers go out. Seeking costs nothing; a Read from somewhere else reopens there.
 func (s *Seeker) Check() error {
 	if s.cur != nil {
 		return nil
@@ -54,11 +54,7 @@ func (s *Seeker) Seek(offset int64, whence int) (int64, error) {
 	if pos < 0 {
 		return 0, errors.New("seek before start")
 	}
-	if s.cur != nil && pos != s.at {
-		s.cur.Close()
-		s.cur = nil
-	}
-	s.pos = pos
+	s.pos = pos // the stream, if open, stays where it is until a Read wants somewhere else
 	return pos, nil
 }
 
@@ -66,6 +62,10 @@ func (s *Seeker) Read(p []byte) (int, error) {
 	if s.pos >= s.size {
 		return 0, io.EOF
 	}
+	if s.cur != nil && s.pos != s.at {
+		s.cur.Close()
+		s.cur = nil
+	}
 	if s.cur == nil {
 		f, err := s.fsys.OpenAt(s.ctx, s.path, s.pos)
 		if err != nil {