The token carries the node's name

Found by raising a mesh end to end. The broker account a joining node
authenticates as is named after the node, and exists before that machine
has been told anything — so the node has to know its name before the mesh
can tell it. Without it, enrolment fails at the broker with an empty
username, which says nothing about why.

Not a secret, and the issuer already knows it. The wire-format test now
covers it, so a rename on either side fails in both repositories rather
than at enrolment on a real machine.

internal/token/token.go

@@ -19,7 +19,7 @@ import (
 	"strings"
 )
 
-// Token is the four things, and it is assembled by whatever holds all four.
+// Token is everything a joining node needs, assembled by whatever holds all of it.
 //
 // Two contexts contribute: `inventory` owns the node record and mints the secret, `identity` owns
 // the signing key. Neither reads the other's store — the process holding both grants asks each
@@ -27,6 +27,17 @@ import (
 type Token struct {
 	Version int `json:"v"`
 
+	// Node is what the mesh calls this machine.
+	//
+	// Not a secret and not the node's to choose — the record was created before the token was
+	// issued, and the broker account the node must authenticate as is named after it. So the node
+	// has to know it *before* the mesh can tell it anything, which is why it travels here.
+	//
+	// It was not here at first, and enrolment then needed a separate flag while its own help said
+	// the token was the only thing required. The failure that produced was a connection refused
+	// with an empty username, which says nothing about the cause.
+	Node string `json:"node,omitempty"`
+
 	// Broker is an address and not a name. There is no resolution before joining, which is why
 	// this is the one place in the mesh where an address is carried deliberately.
 	Broker string `json:"broker,omitempty"`
@@ -50,6 +61,9 @@ type Token struct {
 // a compromised broker could then forge declarations, and that is the whole machine.
 func (t Token) Missing() []string {
 	var missing []string
+	if strings.TrimSpace(t.Node) == "" {
+		missing = append(missing, "the node's name — the broker account is named after it")
+	}
 	if strings.TrimSpace(t.Broker) == "" {
 		missing = append(missing, "the broker's address — there is nowhere to connect to")
 	}