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

script/generate

@@ -0,0 +1,52 @@
+#!/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. A test fails while mf.proto no longer matches the recorded hash.
+#
+# Needs exactly the protoc and protoc-gen-go versions named in the header of
+# the committed mf.pb.go (README.md says how to install them). Another
+# version writes a different mf.pb.go, so the script refuses to run.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# protoc 33.4 names itself v6.33.4 in the mf.pb.go header.
+PROTOC_VERSION="33.4"
+PROTOC_GEN_GO_VERSION="v1.36.11"
+
+# require_version <command> <its exact --version output>
+require_version() {
+    actual="$("$1" --version 2>/dev/null || true)"
+    if [ "$actual" != "$2" ]; then
+        echo "generate: needs $2 on PATH, found: ${actual:-none}" >&2
+        echo "  README.md says how to install it." >&2
+        exit 1
+    fi
+}
+
+# sha256 <file>: print "<hash>  <file>", with sha256sum, or with shasum
+# where there is no sha256sum.
+sha256() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        sha256sum "$1"
+    elif command -v shasum >/dev/null 2>&1; then
+        shasum -a 256 "$1"
+    else
+        echo "generate: needs sha256sum or shasum on PATH" >&2
+        exit 1
+    fi
+}
+
+main() {
+    cd "$ROOT/mfer"
+    require_version protoc "libprotoc $PROTOC_VERSION"
+    require_version protoc-gen-go "protoc-gen-go $PROTOC_GEN_GO_VERSION"
+    # Hashed before regenerating, so a missing hash tool stops the script
+    # before it changes anything. Regenerating leaves mf.proto as it is.
+    proto_hash="$(sha256 mf.proto)"
+    go generate .
+    echo "$proto_hash" >mf.proto.sha256
+}
+
+main "$@"