Compose a node's two logins from the vault's node-logins, so a rotation can switch with both loaded

ADR 0583 §1: a node's account rotates between node.<n> and node.<n>--b, each with the node's permissions and its
inbox _INBOX.node.<n>. Records gains NodeLogins (each node's key of node-logins; the context file's NodeRotating,
renamed because a node that rotated and rotates no more logs in as its second login alone), and Users composes one
node user per login: the one in use and, while it rotates, the other. Each login calls in its own name, which is how
the vault tells the login that confirms (§3). While the vault serves rotate-node, each node reads its own node's key
of node-logins alone and may call node-account-confirm in its own name (§1 insight); the grants part is the state's
one named reader. With no rotation every list is the controller's golden one, unchanged. Tested as a subtest of
TestTheCopiedComposerComposesAsTheControllers, no new test function.

(issue 774, 774/S39)

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/buscompose/buscompose_test.go

@@ -3,6 +3,7 @@ package buscompose
 import (
 	"encoding/json"
 	"errors"
+	"maps"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -419,6 +420,76 @@ func TestTheCopiedComposerComposesAsTheControllers(t *testing.T) {
 			}
 		}
 	})
+	// 774/S39 (ADR 0583 §1): while a node rotates, both its logins are users with the node's permissions and its one
+	// inbox, each calling in its own name; every node reads its own node-logins key alone; with no rotation the list
+	// is the one above (AC2: the controller's golden lists, unchanged).
+	t.Run("a node rotating has two logins of one node", func(t *testing.T) {
+		vault := Seat{Name: VaultSeat, Serves: []string{"rotate-node", "node-account-confirm", "sign", "own"}}
+		since := time.Date(2026, 11, 2, 1, 0, 0, 0, time.UTC)
+		users, err := Users(Records{Nodes: []string{"ace", "g14"},
+			Assigned: map[string][]Declared{"ace": {{Module: "mesh-vault", Holds: []Seat{vault}}}},
+			NodeLogins: map[string]delivery.NodeLogin{
+				"g14": {Node: "g14", Login: "node.g14", Rotating: "node.g14--b", Stage: delivery.RotationBegun, Since: since},
+				// A node that rotated before and rotates no more logs in as its second login alone.
+				"ace": {Node: "ace", Login: "node.ace--b", Since: since}}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		perms := map[string]Permissions{}
+		for _, u := range users {
+			if u.Kind == KindNode {
+				p, err := PermissionsFor(u)
+				if err != nil {
+					t.Fatal(err)
+				}
+				perms[u.Username()] = p
+			}
+		}
+		if len(perms) != 3 || perms["node.g14"].Publish == nil || perms["node.g14--b"].Publish == nil || perms["node.ace--b"].Publish == nil {
+			t.Fatalf("the node users are %v", slices.Sorted(maps.Keys(perms)))
+		}
+		// Equal, but for the caller each names in its calls (the bus's proof of which login called, ADR 0583 §3).
+		own := func(p Permissions, login string) Permissions {
+			swap := func(in []string) []string {
+				out := make([]string, len(in))
+				for i, s := range in {
+					out[i] = strings.ReplaceAll(s, CallerToken(login), "<caller>")
+				}
+				return out
+			}
+			return Permissions{Publish: swap(p.Publish), PublishDeny: swap(p.PublishDeny), Subscribe: swap(p.Subscribe),
+				AllowResponses: p.AllowResponses}
+		}
+		if a, b := own(perms["node.g14"], "node.g14"), own(perms["node.g14--b"], "node.g14--b"); !reflect.DeepEqual(a, b) {
+			t.Errorf("the two logins' permissions differ:\n%v\n%v", a, b)
+		}
+		for login, p := range perms {
+			node, _ := delivery.LoginNode(login)
+			if !slices.Contains(p.Subscribe, "_INBOX.node."+node+".>") || slices.Contains(p.Subscribe, "_INBOX."+login+".>") && login != "node."+node {
+				t.Errorf("%s's inbox: %v", login, p.Subscribe)
+			}
+		}
+		// AC3: g14 reads node-logins key g14 and not ace, and writes neither; it confirms in its own login's name.
+		get := func(key string) string {
+			return "$JS.API.DIRECT.GET.KV_" + delivery.NodeLoginsBucket + ".$KV." + delivery.NodeLoginsBucket + "." + key
+		}
+		confirm := func(login string) string {
+			return "mesh.seat.mesh-vault.call.node-account-confirm." + CallerToken(login)
+		}
+		for subject, want := range map[string]bool{get("g14"): true, get("ace"): false, "$KV." + delivery.NodeLoginsBucket + ".g14": false,
+			confirm("node.g14--b"): true, confirm("node.g14"): false} {
+			if MayPublish(perms["node.g14--b"], subject) != want {
+				t.Errorf("node.g14--b may publish %s: %v, want %v", subject, !want, want)
+			}
+		}
+		if plain, _ := PermissionsFor(Principal{Kind: KindNode, Node: "g14"}); MayPublish(plain, get("g14")) {
+			t.Error("a node reads node-logins while the vault does not rotate")
+		}
+		// And nothing but the grants part names node-logins among its reads.
+		if !ReaderMay(GrantsModule, delivery.NodeLoginsRead) || ReaderMay("shop", delivery.NodeLoginsRead) {
+			t.Error("node-logins' named readers")
+		}
+	})
 	t.Run("the controller's golden server configuration", func(t *testing.T) {
 		seat := Seat{Name: "telegram-sender", Accepts: []string{"send"}, Emits: []string{"delivered", "failed"}}
 		got, err := Compose(