Reposition the temp file when seeking to offset zero

[youdie006]

On the os backend, Seek(0, io.SeekStart) followed by Write appends instead of overwriting from
the start. mem gets it right.

os   Seek(0,0)+Write("HELLO") -> "hello worldHELLO"   want "HELLO world"   FAIL
os   Seek(6,0)+Write("there") -> "hello there"        want "hello there"   PASS

mem  Seek(0,0)+Write("HELLO") -> "HELLO world"
mem  Seek(6,0)+Write("there") -> "hello there"

Offset 6 is the case the conformance suite already covers, which is why this has gone unnoticed.

Cause

copyToLocalTempReader buffers writes into a temp file. When the file exists and Seek/Read ran
first, it copies the original in with io.Copy — leaving the temp file's cursor at EOF — and then
repositions, but only when the cursor is past the start (backend/os/file.go:512):

if f.cursorPos > 0 {
    if _, err := tmpFile.Seek(f.cursorPos, 0); err != nil {

For cursorPos == 0 the reposition is skipped, so the write lands at EOF.

Change

Drop the > 0 guard — the seek is needed for offset zero precisely because io.Copy moved the
cursor.

Tests

Two, both red with only backend/os/file.go reverted:

• A case in the shared DefaultIOTestCases, so mem defines the expected value rather than me
  asserting it. Only os fails:

  --- FAIL: TestIOConformance/Seek_to_start,_Write,_Close,_file_exists
      Seek to start, Write, Close, file exists: expected results that text but got some textthat
  

  mem passes it before and after.
• A table-driven case in the osFileTest suite covering offset 0 and offset 6, since the
  conformance run is behind the vfsintegration tag and would not count toward the coverage gate.

go test ./backend/os/ ./backend/mem/ ./backend/testsuite/ is green, as is
go test -tags vfsintegration -run TestIOConformance ./backend/os/ ./backend/mem/. gofmt -l lists
none of the changed files. CHANGELOG entry added under Unreleased.

Scope

sftp and azure delegate Seek straight through and may share this shape, but I could not
exercise them without credentials, so this PR is limited to os.

Separately and not included: os, azure and sftp do not validate whence (Linux accepts 3
for SEEK_DATA) while mem, s3, gs and ftp return vfs.ErrSeekInvalidWhence
(errors.go:25). Happy to send that separately if you want it.

---

Disclosure: prepared with AI assistance; I verified the backend comparison and both red/green runs
myself.