Rotate a credential and move both ends together

The invariant novox/hq ADR 0001 records as unowned, and it was measurably
false in HAL: a provision documented as never rotating minted a new password on
every adoption and updated only the provider's row. Consumers on three nodes
held dead credentials for two days while the mesh reported success. Nothing
enumerated who held the old one.

Three things make that impossible here. The holders are a set the mesh can name
— each pair has its own credential, so rotating one consumer touches one role
and the affected list is a query rather than an assumption. Both ends are
pushed by this command rather than a later one, because leaving the sending to
whoever remembered is the fault exactly. And it is all-or-nothing: if any
affected machine cannot be resolved, nothing is sent and the old credential
keeps working, which is a mesh that has not rotated rather than one that has
half-rotated.

The window is stated rather than hidden: a role's password changes on the
provider and the file changes on the consumer, and they cannot be simultaneous.

The provisioner now takes its superuser password from the file the mesh wrote,
which is how the mesh delivers one. Passing it through the environment needed a
person in the middle of the one path that exists so there is not one — and put
a superuser password where `docker inspect` prints it.

internal/inventory/secrets.go

@@ -230,3 +230,42 @@ func (i *Inventory) AcceptSecretForModule(ctx context.Context, node, module, nam
 		record.ID, module, name, sealed.ForConsumer, key)
 	return err
 }
+
+// Holder is one end-to-end credential: who gets it and who must create it.
+type Holder struct {
+	Provision string
+	Consumer  string
+	Provider  string
+}
+
+// HoldersOf is every pair sharing a credential for one provision.
+//
+// **The question rotation has to ask, and the one HAL could not.** There, a provision had a single
+// shared credential and rotating it updated the provider's row; nothing enumerated who else held
+// the old one, so three nodes carried dead credentials for two days and the mesh reported success
+// (novox/hq ADR 0001). Here each pair has its own credential, and this is the list that makes
+// "every consumer" a set the mesh can name rather than a hope.
+//
+// Empty consumer means all of them.
+func (i *Inventory) HoldersOf(ctx context.Context, provision, consumer string) ([]Holder, error) {
+	rows, err := i.store.Pool().Query(ctx,
+		`select s.name, c.name, p.name from secret s
+		   join node c on c.id = s.consumer
+		   join node p on p.id = s.provider
+		  where s.name = $1 and ($2 = '' or c.name = $2)
+		  order by c.name, p.name`, provision, consumer)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []Holder
+	for rows.Next() {
+		var h Holder
+		if err := rows.Scan(&h.Provision, &h.Consumer, &h.Provider); err != nil {
+			return nil, err
+		}
+		out = append(out, h)
+	}
+	return out, rows.Err()
+}