brain_edit did its own I/O by path around notes.Edit:
- the pre-edit snapshot used os.ReadFile, which hung on a FIFO note held
open by a writer;
- the rollback (when the provenance stamp fails) used os.WriteFile;
- the edit lock "<note>.lock" was opened with O_CREATE by path, so a
planted dangling .lock symlink made a successful edit create a file at
the link's target, in the brain or outside it.
Snapshot is brainfs ReadRegular, rollback WriteRegular, and the lock is
opened and removed through new brainfs OpenLock/RemoveLock: clean path,
real parent directories, an existing lock must be regular, and the opened
handle must be the file at the path (SameFile).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
