Say each machine's last send in status and node show, and as fields in status --json (novox/hq issue 485)

The send record of issue 234 is now readable only through the controller's
verbs (issue 506), and only plan said it. status says per machine when it was
last sent a declaration, by whom, from which generation and what it answered;
node show says it in full with the refusal's words. A machine's answer is read
from its report only when that report is about the send and came after it.

cmd/mesh-controller/readable.go

@@ -109,6 +109,33 @@ type meshStatus struct {
 	// why they could not be read.
 	Pending       []pendingStatus `json:"pending,omitempty"`
 	PendingUnread string          `json:"pendingUnread,omitempty"`
+	// LastSent is every machine's last recorded send — who sent it, under which lease epoch, from which
+	// assignment generation, naming which modules — and what the machine answered of it (novox/hq issue 485).
+	// A machine the mesh has recorded no send to is absent. LastSentUnread says why it could not be read.
+	LastSent       []machineLastSent `json:"lastSent"`
+	LastSentUnread string            `json:"lastSentUnread,omitempty"`
+}
+
+// machineLastSent is one machine's last send as status says it.
+type machineLastSent struct {
+	Node       string    `json:"node"`
+	Sequence   int64     `json:"sequence"`
+	SentAt     time.Time `json:"sentAt"`
+	Sender     string    `json:"sender"`
+	Epoch      int64     `json:"epoch"`
+	Generation int64     `json:"generation"`
+	Digest     string    `json:"digest"`
+	Modules    []string  `json:"modules"`
+	// Answer is what the machine said of it: applied, failed or refused, as its report has it; empty when it
+	// has not reported on it. Refused is its words when it refused it whole, Failed how many resources failed.
+	Answer     string     `json:"answer"`
+	Refused    string     `json:"refused,omitempty"`
+	Failed     int        `json:"failed,omitempty"`
+	AnsweredAt *time.Time `json:"answeredAt,omitempty"`
+	// RefusedForGeneration is when the machine refused it for coming from an older assignment generation than
+	// it applied, and AppliedGeneration that generation; absent when it did not.
+	RefusedForGeneration *time.Time `json:"refusedForGeneration,omitempty"`
+	AppliedGeneration    int64      `json:"appliedGeneration,omitempty"`
 }
 
 // pendingStatus is one pending assignment as status says it.
@@ -267,6 +294,7 @@ func statusAsJSON(asked answers) ([]byte, error) {
 		out.Pending = append(out.Pending, pendingStatus{Node: p.Node, Module: p.Module, State: p.State,
 			Build: p.Build, Repository: p.Repository, Path: p.Path, Since: p.Since, Settled: p.Settled, Note: p.Note})
 	}
+	out.LastSent, out.LastSentUnread = lastSentStatus(asked.lastSent), asked.lastSentUnread
 	for name := range asked.refused {
 		out.Unresolved = append(out.Unresolved, machineUnresolved{
 			Node: name, Problem: asked.refused[name]})
@@ -323,3 +351,20 @@ func say(value any) error {
 	fmt.Println(string(body))
 	return nil
 }
+
+// lastSentStatus is every machine's last send as status --json says it, by machine name (novox/hq issue 485).
+func lastSentStatus(sends map[string]inventory.Send) []machineLastSent {
+	out := []machineLastSent{}
+	for _, s := range sends {
+		modules := s.Modules
+		if modules == nil {
+			modules = []string{}
+		}
+		out = append(out, machineLastSent{Node: s.NodeName, Sequence: s.Sequence, SentAt: s.SentAt, Sender: s.Sender,
+			Epoch: s.Epoch, Generation: s.Generation, Digest: s.Digest, Modules: modules, Answer: s.Answer.Outcome,
+			Refused: s.Answer.Refused, Failed: s.Answer.Failed, AnsweredAt: s.Answer.At,
+			RefusedForGeneration: s.RefusedAt, AppliedGeneration: s.RefusedApplied})
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Node < out[j].Node })
+	return out
+}