Add the POSIX line terminator on save and strip it on load, so files
written by the editor no longer trip git's "No newline at end of file".
Done at the persistence choke point, not in :fmt: the editor buffer is
newline-free by design (rows = #\n + 1), so a trailing '\n' in the buffer
would render a phantom blank last line. save_path adds exactly one '\n'
(guarded against doubling); load_path strips one back off so the buffer
stays newline-free and round-trips byte-stable.
Update the sd_fat spike payload to be newline-free so its byte-identity
round-trip assertion holds under the new normalizing contract.
82f305cea6