Serve a key's kept values so the issue tracker can carry them into its log (ADR 0307, issue 502)

mesh/state.history answers every value a bucket still keeps for a key,
oldest first, to whoever may get it, by the same watch the runtime already
uses, so no new grant is needed.

node-tools/internal/bus/state.go

@@ -129,6 +129,50 @@ func (c *Conn) StateGet(module, name, key string) (*StateEntry, error) {
 	return &StateEntry{Key: e.Key(), Value: valueOf(e.Value()), Revision: e.Revision()}, nil
 }
 
+// StateKept is one value a bucket still keeps for a key: a put, or a delete (a purge reads as one).
+type StateKept struct {
+	Key      string          `json:"key"`
+	Value    json.RawMessage `json:"value,omitempty"`
+	Revision uint64          `json:"revision"`
+	Op       string          `json:"op"`
+	At       time.Time       `json:"at"`
+}
+
+// StateHistory is every value the bucket still keeps for one key, oldest first, or none (novox/hq ADR
+// 0307). It is a read like StateGet: served to the owner and to every reader, and writes nothing.
+func (c *Conn) StateHistory(module, name, key string) ([]StateKept, error) {
+	s, err := c.issuedState(module, name)
+	if err != nil {
+		return nil, err
+	}
+	if err := checkKey(key); err != nil {
+		return nil, err
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), StateTimeout)
+	defer cancel()
+	kv, err := c.bucket(ctx, s)
+	if err != nil {
+		return nil, err
+	}
+	entries, err := kv.History(ctx, key)
+	kept := []StateKept{}
+	if errors.Is(err, jetstream.ErrKeyNotFound) {
+		return kept, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	for _, e := range entries {
+		k := StateKept{Key: e.Key(), Revision: e.Revision(), Op: "delete", At: e.Created().UTC()}
+		if e.Operation() == jetstream.KeyValuePut {
+			k.Op = "put"
+			k.Value = valueOf(e.Value())
+		}
+		kept = append(kept, k)
+	}
+	return kept, nil
+}
+
 // StatePut writes one key, as the module, where the module keeps the state. It answers the revision.
 func (c *Conn) StatePut(module, name, key string, value json.RawMessage) (uint64, error) {
 	s, err := c.writable(module, name)