Deleting a specific version that happens to be the latest needs the new latest
re-derived from the remaining versions, and that scan must run under the same
lock as the delete. The gateway can't do it atomically across RPCs.
Add a RECOMPUTE_LATEST mutation: it scans a directory under the transaction
lock, picks the child that sorts last (descending) or first by name, copies the
mapped extended keys from it into a pointer entry, and stores its name under
name_to_key. An empty directory clears the pointer keys. The filer stays
mechanical and S3-agnostic: the caller, which knows the versioning scheme,
supplies the sort direction and the key mappings. A missing pointer entry is a
no-op, so a replayed transaction is idempotent.
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