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

Makefile

@@ -2,7 +2,6 @@ export DOCKER_BUILDKIT := 1
 export PROGRESS_NO_TRUNC := 1
 GOPATH := $(shell go env GOPATH)
 export PATH := $(PATH):$(GOPATH)/bin
-PROTOC_GEN_GO := $(GOPATH)/bin/protoc-gen-go
 SOURCEFILES := mfer/*.go mfer/*.proto internal/*/*.go cmd/*/*.go go.mod go.sum
 ARCH := $(shell uname -m)
 GITREV_BUILD := $(shell bash $(PWD)/bin/gitrev.sh 2>/dev/null || echo unknown)
@@ -13,7 +12,7 @@ GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Gitrev=$(GITREV_BUILD)
 GOFLAGS := -ldflags "$(GOLDFLAGS)"
 
-.PHONY: bootstrap setup docker default run ci test fuzz check lint fmt fmt-check fmt-check-go fmt-check-md hooks fixme
+.PHONY: bootstrap setup docker default run ci test fuzz check lint fmt fmt-check fmt-check-go fmt-check-md hooks fixme generate
 
 default: fmt test
 
@@ -35,9 +34,6 @@ test:
 fuzz:
 	@script/fuzz
 
-$(PROTOC_GEN_GO):
-	test -e $(PROTOC_GEN_GO) || go install -v google.golang.org/protobuf/cmd/protoc-gen-go@v1.28.1
-
 fixme:
 	@grep -nir fixme . | grep -v Makefile
 
@@ -58,15 +54,14 @@ fmt-check-md:
 hooks:
 	@script/install-precommit
 
-mfer/mf.pb.go: mfer/mf.proto
-	cd mfer && go generate .
+generate:
+	@script/generate
 
-bin/mfer: $(SOURCEFILES) mfer/mf.pb.go
-	protoc --version
+bin/mfer: $(SOURCEFILES)
 	cd cmd/mfer && go build -tags urfave_cli_no_docs -o ../../bin/mfer $(GOFLAGS) .
 
 clean:
-	rm -rfv mfer/*.pb.go bin/mfer cmd/mfer/mfer *.dockerimage
+	rm -rfv bin/mfer cmd/mfer/mfer *.dockerimage
 
 fmt:
 	@script/fmt