0039: the link already exists, and most of the cost is already paid

Written first as though the link were a thing to build. It is not. ADR 0001
already has it — every node connects outbound to a single broker, nothing ever
connects to a node, each node declaring an exchange named for itself and
consuming from its own queue. Already outbound-only, already per-node addressed,
already the one channel everything arrives through.

So this record is not proposing a channel. It proposes that the channel carry
per-node identity instead of one shared credential. The same as-is records the
fault it fixes, for the broker rather than the database: the broker's credential
is mesh-wide, rotating it is a mesh-wide operation, and doing it wrong has taken
the broker down.

That reframes the overhead objection, which was fair against what the record
said and not against what it means. Of six properties, four are already true,
one is broker configuration — users, vhosts and per-queue permissions the broker
already implements — and exactly one is new machinery: enrolment. Meanwhile 0037
subtracts, since a node under it holds no database credential at all. Three
hand-carried shared secrets become one identity that grants only identity.

Adds the option that was actually being weighed and was missing: accept the
exposure as the cost of simplicity. Rejected because the simplicity IS the
unfixability — the credential cannot be rotated precisely because everything
holds the same one.

And adds a requirement from the debugging objection, which was the strongest
part of it: it must fail legibly. A boundary that refuses a node without saying
why is worse than the password it replaced, because a wrong password at least
announces itself. That is §5 applied to a security mechanism.

02-DECISIONS/0039-the-link-is-the-security-boundary.md

@@ -41,13 +41,35 @@ done, it has been done by hand, and doing it wrong has taken services down."*
 Compromise of any node is therefore compromise of the mesh's database, and there is no
 mechanism to recover from it.
 
+### The link is not new
+
+Written first as though the link were a thing to build. It is not.
+[ADR 0001](0001-nodes-communicate-over-a-broker.md) already has it: *every node connects
+outbound to a single broker; nothing ever connects to a node*, each node declaring an exchange
+named for itself and consuming from its own queue
+([`00-as-is/01`](../03-DESIGN/00-as-is/01-mesh-and-transport.md)).
+
+That is already outbound-only, already per-node addressed, and already the one channel
+everything arrives through. **This record is not proposing a channel. It is proposing that the
+channel carry per-node identity instead of one shared credential.**
+
+The same as-is records the fault, for the broker rather than the database: *"the broker is a
+single point of failure and a single point of trust. Its credential is mesh-wide, so rotating
+it is a mesh-wide operation, and doing it wrong has taken the broker down."*
+
 ## Considered options
 
 1. **Keep shared credentials, scope them per node.** Least change: give each node its own
    database role. Rejected — it makes the blast radius smaller without changing its shape, and
    it keeps tier 0 speaking the control plane's schema, which ADR 0037 forbids for reasons that
    are not about security at all.
-2. **Mutual authority on a node-initiated link, with the node holding nothing but its own
+2. **Accept the exposure as the cost of simplicity.** A shared credential is one thing to
+   understand and nothing to build, and the objection to replacing it is real: mutual
+   authentication fails opaquely, and a node that cannot link is harder to debug than a node
+   with a wrong password. Rejected on the ground that the simplicity is what makes it
+   unrotatable — the credential cannot be changed *because* everything holds the same one, so
+   the arrangement's convenience and its unfixability are the same property.
+3. **Mutual authority on a node-initiated link, with the node holding nothing but its own
    identity.** Chosen.
 
 ## Decision
@@ -102,6 +124,30 @@ exchange, and it expires whether used or not.
 This replaces hand-carried shared secrets with a thing that is useless once used and useless
 after a while.
 
+## What this actually costs
+
+The objection to weigh is overhead, and it is smaller than it looks because most of it is
+already running.
+
+| Property | Where it comes from |
+|---|---|
+| outbound, node-initiated | already true — ADR 0001 |
+| per-node addressing | already true — per-node exchange and queue |
+| per-node credential | a broker user per node; the broker already has users, virtual hosts and per-queue permissions |
+| mutual authority | transport-level certificates on a connection that already exists |
+| bounded by form | already true — three message shapes and only three |
+| **enrolment** | **the one genuinely new mechanism** |
+
+And ADR 0037 subtracts rather than adds: under it a node holds **no** database credential at
+all, so this record replaces three hand-carried shared secrets with one per-node identity that
+grants only identity.
+
+**It must fail legibly.** A boundary that refuses a node without saying why is worse than the
+credential it replaced, because a wrong password at least announces itself. A node that cannot
+link must report which side rejected it and on what grounds, in terms someone can act on. This
+is `how-we-build` §5 applied to a security mechanism: a refusal that proves only that something
+went wrong is transport reported as effect.
+
 ## Consequences
 
 - **ADR 0037 removes a standing exposure as a side effect.** Its rule — the host never queries