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/generate

@@ -0,0 +1,17 @@
+#!/bin/sh
+# script/generate: regenerate mfer/mf.pb.go from mfer/mf.proto, and record
+# the hash of that mf.proto in mfer/mf.proto.sha256. Nothing else
+# regenerates mf.pb.go: it is committed, so building and checking need no
+# protoc. Needs protoc and protoc-gen-go on PATH. A test fails while
+# mf.proto no longer matches the recorded hash.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT/mfer"
+    go generate .
+    shasum -a 256 mf.proto >mf.proto.sha256
+}
+
+main "$@"