Say the node tools are there, every minute (hq to-be 45 Phase 1, S11)

A machine whose node-engine is heard and whose runtime is gone is a
machine nobody can ask anything, and nothing said so. The runtime now
says on mesh.control.<node>.tools-alive, every minute, that it is there,
with its interval and build; the controller raises tools-silent after
three missed. Core NATS, like the host's heartbeat: a lost one is the
next one. A heartbeat the bus refuses is logged when that starts and
when it stops.

node-tools/internal/alive/alive.go

@@ -0,0 +1,80 @@
+// Package alive is the node tools saying they are there (novox/hq to-be 45 §3, S11).
+//
+// **A machine heard and its runtime gone is a machine nobody can ask anything.** The node-engine says
+// every minute that the machine is there; nothing said whether the runtime serving every module's
+// tools and every held seat's verbs was. Now it says so too, on its own subject, every interval, with
+// the interval — the controller's watchdog raises `tools-silent` after three missed — and its build.
+// Core NATS, kept by nobody, as the host's heartbeat: a lost one is the next one.
+package alive
+
+import (
+	"encoding/json"
+	"runtime/debug"
+	"sync"
+	"time"
+)
+
+// Every is how often the runtime says it is there.
+const Every = 60 * time.Second
+
+// Subject is where one machine's node tools say it.
+func Subject(node string) string { return "mesh.control." + node + ".tools-alive" }
+
+// Beat is what is said: the machine, the interval, and the build.
+type Beat struct {
+	Node            string `json:"node"`
+	IntervalSeconds int    `json:"interval_seconds"`
+	Version         string `json:"version,omitempty"`
+}
+
+// Sayer publishes one message, kept by nobody.
+type Sayer interface {
+	Say(subject string, body []byte) error
+}
+
+// Version is this build's commit, as the toolchain stamped it; empty when it did not.
+func Version() string {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		return ""
+	}
+	for _, s := range info.Settings {
+		if s.Key == "vcs.revision" {
+			return s.Value
+		}
+	}
+	return ""
+}
+
+// Keep says the runtime is there now and every interval until the returned function is called. A
+// heartbeat that cannot be said is logged when that starts and when it stops — not every minute.
+func Keep(bus Sayer, node string, every time.Duration, logf func(string, ...any)) func() {
+	body, _ := json.Marshal(Beat{Node: node, IntervalSeconds: int(every / time.Second), Version: Version()})
+	done := make(chan struct{})
+	var once sync.Once
+	go func() {
+		failing := ""
+		tick := time.NewTicker(every)
+		defer tick.Stop()
+		for {
+			why := ""
+			if err := bus.Say(Subject(node), body); err != nil {
+				why = err.Error()
+			}
+			if why != failing {
+				if why != "" {
+					logf("node-tools: could not say this runtime is there — the mesh will call it silent: %s", why)
+				} else {
+					logf("node-tools: says again that this runtime is there")
+				}
+				failing = why
+			}
+			select {
+			case <-done:
+				return
+			case <-tick.C:
+			}
+		}
+	}()
+	return func() { once.Do(func() { close(done) }) }
+}