A provision names its engine; the substrate supplies only the control plane

**0027 — provisions.** A module written against PostgreSQL could be
matched to a provider of SQL Server, resolve as satisfied, and fail on
its first query. The name said the role, so nothing distinguished
engines. Refusing on ambiguity could not help: with one provider of
each name nothing is ambiguous. Enforced at parse rather than
documented, because the old naming was the documentation.

**0028 — the substrate.** 0006 admits an object store on the grounds
that it cannot grant itself a bucket. That answers the second half of
the test and assumes the first: the control plane does not need one.
Verified — no S3 client in mesh-control, and internal/builder/registry.go
records the deliberate choice to put artifacts in the OCI registry as
content-addressed blobs. The row was inherited from the system being
replaced, where an object store distributed module tarballs, and was
never re-tested against the definition above it.

So an object store is an ordinary module, and a mesh with nothing
needing one runs none. Migrating it is module work, not substrate work.

0028 also states what 0006 left unsaid: a substrate service and a
module of the same product are different instances. The substrate is
raised from the bundle before any mesh exists, so it is not in the
module graph — a workload depending on it would depend on something the
graph cannot see, cannot rotate a credential for, and cannot move, and
would put workload data in the store the control plane keeps its own
state in.

Both records were found by reading code against design rather than
design against itself, which is the review that should have happened
sooner.

02-DECISIONS/README.md

@@ -92,6 +92,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0006** — [The substrate and the control plane](0006-the-substrate-and-the-control-plane.md)
 - **0007** — [Connectivity](0007-connectivity.md)
 - **0008** — [A context owns its store, exclusively](0008-a-context-owns-its-store.md)
+- **0028** — [The substrate supplies the control plane and nothing else](0028-the-substrate-supplies-the-control-plane-and-nothing-else.md)
 
 ### What runs on them, and how it gets there
 
@@ -99,6 +100,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0010** — [Delivery](0010-delivery.md)
 - **0024** — [Model access is a provision, and a licence is a thing with a name](0024-model-access-is-a-provision.md)  *(proposed)*
 - **0026** — [The mesh has a session of its own, and it is the node session's mechanism](0026-the-mesh-has-a-session-of-its-own.md)
+- **0027** — [A provision names what the consumer is coupled to, not the role it plays](0027-a-provision-names-what-the-consumer-is-coupled-to.md)
 
 ### How it is built