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

mfer/mf_test.go

@@ -8,6 +8,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"sneak.berlin/go/mfer/mfer"
 )
 
 // mf.pb.go is generated from mf.proto and committed. `make generate`
@@ -27,3 +29,26 @@ func TestGeneratedCodeMatchesProto(t *testing.T) {
 		"mfer/mf.proto has changed since mfer/mf.pb.go was generated "+
 			"from it: run `make generate` and commit the result")
 }
+
+// A file entry has exactly the fields docs/FORMAT.md lists for MFFilePath.
+func TestFileEntryFieldsMatchSpec(t *testing.T) {
+	t.Parallel()
+
+	want := map[string]protoreflect.FieldNumber{
+		"path":     1,
+		"size":     2,
+		"hashes":   3,
+		"mimeType": 301,
+		"mtime":    302,
+		"ctime":    303,
+	}
+
+	got := map[string]protoreflect.FieldNumber{}
+
+	fields := (&mfer.MFFilePath{}).ProtoReflect().Descriptor().Fields()
+	for i := range fields.Len() {
+		got[string(fields.Get(i).Name())] = fields.Get(i).Number()
+	}
+
+	require.Equal(t, want, got)
+}