Thin UTF-8 writer used by the new save-dialog path. Caller owns path
safety; the source path is always OS-dialog-provided. Two unit tests:
roundtrips a small UTF-8 string with non-ASCII chars and asserts a
nonexistent parent path returns an actionable error.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>