One implementation, several surfaces, and what that costs

The mesh is operated from a command line and must be operable from a
browser and from a model's tools, without becoming three systems. The
pattern is already in the code and was unnamed: `board` serves HTTP by
calling the same functions the CLI calls, holding nothing.

Takes the decision 0034 said had to be taken deliberately rather than
arrive with a feature: the HTTP surface is not read-only, so a browser
login now carries authority over the mesh.

Names the dependency by protocol — an OAuth2 identity provider — as the
mesh does for AMQP, S3 and OCI. Keycloak is what fills the role; what
the control plane knows is that it validates a token, and replacing the
provider is a migration rather than a redesign.

Says what this must not become, because it is the failure the project
was started over: a kernel every module imports, 155 files of code from
every context. Shared surfaces are not a shared library. Three adapters
calling the same functions is not the same as logic leaving the context
that owns it.

And records the loop it creates. The networked surfaces depend on a
module the control plane assigns, so when identity is down nobody can
authenticate — including whoever is trying to fix it. The way out is the
command line, which authenticates through nothing and is available to
the account that owns the machine. Hence the rule: no capability exists
only behind an authenticated surface, because that is a capability which
disappears exactly when identity does.

02-DECISIONS/README.md

@@ -105,6 +105,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0024** — [Model access is a provision, and a licence is a thing with a name](0024-model-access-is-a-provision.md)
 - **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)
+- **0035** — [One implementation, several surfaces, and what that costs](0035-one-implementation-several-surfaces.md)
 
 ### How it is built