std: fix File.Writer sendfile with buffered contents

* File.Writer.seekBy passed wrong offset to setPosAdjustingBuffer.
* File.Writer.sendFile incorrectly used non-logical position.

Related to 1d764c1fdf04829cec5974d82cec901825a80e49

Test case provided by:

Co-authored-by: Kendall Condon <goon.pri.low@gmail.com>
4c27eeff8eAndrew Kelley committed on 10/8/2025, 11:43:42 PMparent8964737
2 files changedLine totals unavailable