Add the checking program: it answers the verdict of a check's facts (issue 1921)

mesh-checking-program -verdict reads the facts from the file the build
seat's run step names, judges them with the rules package and answers
one versioned line; facts of a version it does not read are refused,
naming it, because the running checker of B0 runs G's programs
(milestone 47/B0, ADR 0644, ADR 0656).

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-delivery-checker/cmd/mesh-checking-program/main.go

@@ -0,0 +1,59 @@
+// Command mesh-checking-program is the checker's checking program (novox/hq ADR 0644, ADR 0656; design 47's bootstrap
+// delivery): built from a check's heads and run in the build seat's sandbox through the seat node-build-go's ask run,
+// with no network and no bus account, it reads the check's facts from the file MESH_RUN_INPUT names and answers the
+// verdict the judging rules give them, as one JSON line on its standard output (rules.Answer).
+//
+//	mesh-checking-program -verdict
+//
+// It refuses facts of a version it does not read, naming it, with exit 2.
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"mesh-delivery-checker/rules"
+)
+
+func main() {
+	os.Exit(run(os.Args[1:], os.Getenv, os.Stdout, os.Stderr))
+}
+
+// run is the command over its arguments, environment and outputs, answering its exit.
+func run(args []string, getenv func(string) string, stdout, stderr io.Writer) int {
+	fs := flag.NewFlagSet("mesh-checking-program", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	verdict := fs.Bool("verdict", false, "answer the verdict of the facts in the input")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+	refuse := func(format string, a ...any) int {
+		fmt.Fprintf(stderr, "mesh-checking-program: "+format+"\n", a...)
+		return 2
+	}
+	if !*verdict {
+		return refuse("nothing asked: give -verdict")
+	}
+	file := getenv(delivery.RunInputEnv)
+	if file == "" {
+		return refuse("no input: %s names no file", delivery.RunInputEnv)
+	}
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return refuse("the input cannot be read: %v", err)
+	}
+	facts, err := rules.DecodeFacts(data)
+	if err != nil {
+		return refuse("%v", err)
+	}
+	line, err := json.Marshal(rules.Answer{V: rules.AnswerVersion, Verdict: rules.Judge(facts)})
+	if err != nil {
+		return refuse("the verdict cannot be written: %v", err)
+	}
+	fmt.Fprintf(stdout, "%s\n", line)
+	return 0
+}