Define the substrate, and answer 006's four-or-five conditionally

Same gap as the control plane: load-bearing and unpinned.

The substrate is what the control plane CONSUMES AND CANNOT GRANT ITSELF. Every
module needing a database asks provisioning for one; the control plane needs one
too and cannot ask itself, because it is not running yet. That circularity is
not an awkwardness to work around — it is the definition, and anything on the
wrong side of it must be raised by the bundle the host carries.

Which answers 006's open question in the honest form rather than with a number.
The identity provider is substrate only if the control plane DELEGATES
authentication — then it cannot serve anybody before the provider exists and
cannot grant itself a client. If it authenticates natively, the provider is an
ordinary hosted service. So the count follows from a decision not yet taken, and
asserting four was asserting that decision.

The test also rules out the tempting wrong answer: an identity provider, a mail
server and an analytics service are all infrastructure by any ordinary reading,
and none are substrate, because the control plane starts and runs without them.
Important is not the test.

Records why the bundle is pinned by hand — it is applied when no mesh exists, so
nothing can resolve a version or ask a registry — and why it must be
self-contained, which makes it an artifact built on a machine with a network for
a machine that may have none.

01-RESEARCH/006-mesh-from-scratch/00-overview.md

@@ -90,5 +90,5 @@ the catalogue where modules genuinely change together under one intent. The skel
 | One repository per tier, or per context? | Already open from ADR 0015 as "catalogue destination — one repository or many". The skeleton assumes per tier and does not settle it. |
 | ~~Does an unprivileged node earn a place in the inventory, or only a presence?~~ | **Answered 2026-08-25** by the operator: a node is a *managed machine inside the mesh*, not an unprivileged something — and a disconnected node is still a node, in a different situation. The question posed a class distinction; the answer is that there is none, and what varies is **state**. Recorded as [ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md). |
 | ~~Does absorbing overlay, filtering, packages, supervision and the container runtime make the host too large?~~ | **Answered 2026-08-25** — [`host-size.md`](host-size.md). Measured: the absorption is smaller than the machinery that already applies state, and eight of ten adapters already carry no dependency. The risk is not size but direction, and it is two modules wide. The claim survives with its scope corrected — the host carries one concern, *apply declared state on this machine*, of which the six are instances. Recorded as [ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md), designed in [`05-the-node-host.md`](../../03-DESIGN/01-to-be/05-the-node-host.md). |
-| Four substrate services or five? | The identity provider passes the tier test only if the control plane delegates authentication rather than doing it natively. |
+| ~~Four substrate services or five?~~ | **Answered conditionally**, which is the honest form — [`07-the-substrate.md`](../../03-DESIGN/01-to-be/07-the-substrate.md). The substrate is *what the control plane consumes and cannot grant itself*. The identity provider qualifies only if the control plane delegates authentication; if it authenticates natively it is an ordinary hosted service. The count follows from a decision not yet taken, and asserting four was asserting that decision. |
 | Does `feature` survive? | The skeleton splits it in two and argues the conflation is what makes the delivery pipeline hard to reason about. Unproven. |