Add Plan Atomic Commit, and reverse Write Durability's recommendation The durability page landed on a journal plus signal blocking, and treated RCS's temp-file-and-rename as the expensive option. It was missing the fact that decides the question: RCS rewrites the whole ,v because it stores HEAD first, so a commit shifts everything after it. We store HEAD last, so the unchanged part is a prefix -- copied as raw bytes, no parsing, no re-serialisation. That removes both objections at once, the O(file) cost (fsync dominates at wiki scale) and the bit-flip exposure of §6. So the new page is what was built, and §8 of the old one now says it was overturned rather than quietly disagreeing with the code. The journal and the signal blocking are dropped: with no write over live data there is nothing to undo and no Ctrl-C window. The page also records where the plan was wrong -- sendfile is Linux-only for file-to-file, abspath does not resolve symlinks, and the flush it argued for turned out to be unreachable -- since those are the parts worth remembering.