Never regenerate mf.pb.go during checks; fail when it is stale (closes #71)

The test and format scripts regenerated mfer/mf.pb.go whenever
mfer/mf.proto looked newer by mtime, which a fresh checkout often causes,
so make check could rewrite a committed file and needed protoc. Nothing
regenerates it any more except make generate (script/generate), which
refuses to run unless protoc 33.4 and protoc-gen-go v1.36.11, the
versions that wrote the committed file, are on PATH, and records the
hash of mf.proto in mfer/mf.proto.sha256. A Go test compares that hash
with mf.proto and fails, naming make generate, when they differ. The
mtime rule, the unused protoc-gen-go v1.28.1 install rule, make clean's
deletion of mf.pb.go and the Dockerfile's touch workarounds are removed.

Model: opus-5-5

mfer/mf.proto.sha256

@@ -0,0 +1 @@
+103901c42b94396aa7ae128fd503ef693a4b7a03b2169481f25fda3d2c254e00  mf.proto

mfer/mf_test.go

@@ -0,0 +1,29 @@
+package mfer_test
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// mf.pb.go is generated from mf.proto and committed. `make generate`
+// records the hash of the mf.proto it generated from in mf.proto.sha256.
+func TestGeneratedCodeMatchesProto(t *testing.T) {
+	t.Parallel()
+
+	proto, err := os.ReadFile("mf.proto")
+	require.NoError(t, err)
+
+	recorded, err := os.ReadFile("mf.proto.sha256")
+	require.NoError(t, err)
+
+	recordedHash, _, _ := strings.Cut(string(recorded), " ")
+	sum := sha256.Sum256(proto)
+	require.Equal(t, recordedHash, hex.EncodeToString(sum[:]),
+		"mfer/mf.proto has changed since mfer/mf.pb.go was generated "+
+			"from it: run `make generate` and commit the result")
+}