The tail reader's no-trailing-newline branch (a torn/truncated final
append, the case M3's fsync defends) and the over-window error branch
had no coverage. Add cases for a newline-free last line, a torn final
append that must be rejected, and a single line exceeding the 64KiB
window.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>