Drop atime from the format spec and pin the file entry fields (closes #158)

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 against the
fields the spec lists, by name and number, so a field added to or
removed from the proto without a matching spec change fails the build.

Model: opus-5-5

docs/FORMAT.md

@@ -78,9 +78,6 @@ Each file entry contains:
 | `mtime`    | 302    | Timestamp (optional)      | Modification time                   |
 | `ctime`    | 303    | Timestamp (optional)      | Change time (inode metadata change) |
 
-Field 304 (`atime`) has been removed from the specification. Access time is
-volatile and non-deterministic; it is not useful for integrity verification.
-
 ## Path Rules
 
 All `path` values must satisfy these invariants:
@@ -130,7 +127,6 @@ By default, manifests are generated deterministically:
 
 - File entries are sorted by `path` in **lexicographic byte order**
 - `createdAt` is omitted unless explicitly requested
-- `atime` is never included (field removed from schema)
 
 This ensures that two independent runs over the same directory tree produce
 byte-identical `.mf` files (assuming file contents and metadata have not