Genesis can raise a mesh on the new bus, and the carried user list is checked against the composer

The mesh writes its own user list, and at genesis there is no mesh yet to write it. So
the installer carries the first one — the controller's own account at a well-known
bootstrap password, exactly as the store is reached at `postgres:bootstrap` and the old
bus at `guest:guest`, and rotated with them. From the controller's first composition
onward the file is the controller's.

That left a gap I would not have found by reading: the controller's own account is
created before there is a controller to mint one, so nothing recorded a hash for it, and
its first composition would have left the writer out of the file it was writing — a bus
nothing can connect to, produced by the thing connected to it. It now records a hash of
the credential it is actually using, and only if none is recorded, so a restart cannot
put the bootstrap password back over a rotated one.

The carried list and the derived one are two statements of one fact, so a test compares
them: every subject the controller derives must be in the template, and nothing wider.
It earned itself immediately — the composer was granting both a role's whole event
branch and the one event it actually follows, which is a wider way of saying the same
thing, and the wider one wins. Only the submitting half of a role is granted now; what
comes back is named exactly.

Getting this wrong is the worst kind of silent. A controller whose carried permissions
are narrower than the ones it derives comes up, connects, and is refused on the first
thing it tries, with an authorisation error naming a subject and not the template that
forgot it — and a mesh cannot be raised twice to find out.

internal/inventory/bususers.go

@@ -136,3 +136,30 @@ func (i *Inventory) ForgetBusUsersOf(ctx context.Context, node string) error {
 	_, err := i.store.Pool().Exec(ctx, `delete from bus_user where node = $1`, node)
 	return err
 }
+
+// SeedBusUser records a hash of a credential the mesh did not mint, so a composition contains it.
+//
+// **Genesis is the reason this exists.** The controller's own user is created before the controller
+// runs — by the installer, at a well-known bootstrap password, the way the store's and the old bus's
+// are (`postgres:bootstrap`, `guest:guest`). Nothing minted it, so nothing recorded a hash for it, and
+// the controller's first composition would leave itself out of the very file it was writing: a bus
+// nothing can connect to, produced by the thing connected to it.
+//
+// Idempotent, and it does not overwrite. A credential the mesh *did* mint is the one that counts, so
+// once there is a row this does nothing — otherwise a restart would put the bootstrap password back
+// over a rotated one.
+func (i *Inventory) SeedBusUser(ctx context.Context, u BusUser, password string) error {
+	if u.Username == "" || u.Kind == "" || password == "" {
+		return errors.New("a bus user needs a username, a kind and the credential it is using")
+	}
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return fmt.Errorf("cannot hash a bus password: %w", err)
+	}
+	_, err = i.store.Pool().Exec(ctx,
+		`insert into bus_user (username, kind, node, module, password_hash)
+		 values ($1, $2, $3, $4, $5)
+		 on conflict (username) do nothing`,
+		u.Username, u.Kind, u.Node, u.Module, string(hash))
+	return err
+}