atime left mf.proto earlier and nothing reads or writes it, but
docs/FORMAT.md still narrated its removal and listed it among the
determinism rules. The spec now describes the file entry by its fields
only.
A new test compares the MFFilePath message descriptor, by name and
number, with a list copied from the spec's field table. It does not read
the spec, so changing a field means changing the proto, the spec and
that list together.
Model: opus-5-5
sneak