Closes the last open question about what the substrate contains. 0006
left an identity provider conditional — substrate only if the control
plane delegated authentication — and said the decision had not been
taken. It is now: it delegates to nothing.
The conditional was never about machines. A node proves itself with a
keypair it generated over a broker account issued at enrolment, and
declarations are verified by signature; none of that involves an
identity provider. It was only ever about whether a person signing in to
a mesh surface would be authenticated by something else.
So the substrate is three — a relational store, a message bus, an image
registry — and with 0028 having removed the object store, no member is
conditional and every one is there for the same reason.
It does not settle how a person signs in to a surface, deliberately.
What is settled is that whatever answers that is not something which
must exist before the mesh does, so it can be decided late or replaced —
which being substrate would have prevented.
