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
also 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; it needs no protoc. The Makefile's mtime rule for mf.pb.go,
make clean's deletion of it and the Dockerfile's touch workarounds are
removed.

Model: opus-5-5

script/test

@@ -4,19 +4,8 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Regenerate mfer/mf.pb.go from mfer/mf.proto if it is missing or stale
-# (mirrors the old Makefile prerequisite; the generated file is
-# committed, so this is normally a no-op).
-ensure_pb() {
-    if [ ! -f mfer/mf.pb.go ] ||
-        [ -n "$(find mfer/mf.proto -newer mfer/mf.pb.go 2>/dev/null)" ]; then
-        (cd mfer && go generate .)
-    fi
-}
-
 main() {
     cd "$ROOT"
-    ensure_pb
     go test -timeout 30s -race -cover ./... ||
         {
             echo "--- Rerunning with -v for details ---"