Retry an apply, a fetch by digest and a have write that met the bus reloading, and keep the cause in the have

The bus node's engine writes the bus user list (ADR 0547 bridge); each write makes the bus reload its accounts, and
for about three seconds every JetStream request meets no responders. An apply that failed then was kept failed until a
key changed (mesh-vault on novox, 2026-10-25 23:04:24), holding every module that needs the secret seat. A transient
bus failure is now retried with a backoff (2 s, 5 s, 15 s, 60 s, then every 5 min), the have saying retrying meanwhile;
after the engine writes the user list, the transiently failed modules are tried again once the reload is over. A have
write that met it is written again the same way. shorten keeps the head and the tail, the cause.

cmd/mesh-host/userlist.go

@@ -23,7 +23,7 @@ import (
 // userListFollower is the follower of the bus's user list for one link: the node's composition bucket, the keys the
 // agent trusts, the write alone, and the have.
 func userListFollower(opts options, session *link.Session, composedBucket string, trusted func() []ed25519.PublicKey,
-	say link.Announce) *userlist.Follower {
+	say link.Announce, wrote func()) *userlist.Follower {
 	return userlist.New(userlist.Env{
 		Node:     session.Membership.Node,
 		Composed: bucketWatch{session, composedBucket},
@@ -33,7 +33,11 @@ func userListFollower(opts options, session *link.Session, composedBucket string
 		},
 		OnDisk: userListOnDisk,
 		Write: func(ctx context.Context, module string, raw json.RawMessage, body []byte) (string, bool, error) {
-			return writeUserListAlone(session.WithLink(ctx), opts, raw, body, say)
+			digest, changed, err := writeUserListAlone(session.WithLink(ctx), opts, raw, body, say)
+			if changed && wrote != nil {
+				wrote()
+			}
+			return digest, changed, err
 		},
 		Report: func(ctx context.Context, module string, w userlist.Written) {
 			session.NoteUserList(ctx, module, w)