Divide the bus work into five steps, each proved on its own

The NATS change was recorded as one undivided item, which hid three gaps:
a mesh already running had no adoption path, the protocol specification did
not know its transport was being replaced, and nothing was runnable until
everything was. Dividing it is what surfaced them.

03-DESIGN/01-to-be/19-the-module-protocol.md

@@ -5,9 +5,11 @@ code:
   - mesh-sdk src
   - mesh-tools src/broker-amqp.ts
   - mesh-controller internal/link
-updated: 2026-09-21
+updated: 2026-09-26
 decisions:
   - 02-DECISIONS/0095-the-control-plane-is-the-way-to-ask-a-module.md
+  - 02-DECISIONS/0106-the-bus-is-nats.md
+  - 02-DECISIONS/0115-the-bus-is-built-in-five-steps.md
   - 02-DECISIONS/0074-the-wire-is-specified-not-the-types.md
   - 02-DECISIONS/0039-what-the-sdk-holds-and-refuses.md
   - 02-DECISIONS/0043-a-module-broker-account-is-scoped-by-emits-and-consumes.md
@@ -23,6 +25,27 @@ language and nothing more ([ADR 0074](../../02-DECISIONS/0074-the-wire-is-specif
 This is a specification, so it says what is required rather than how anything is arranged. Where it
 describes current behaviour that is *not yet* specified-and-conformed, it says so.
 
+> **The wire below is the bus being replaced.** *2026-09-26.*
+> [ADR 0106](../../02-DECISIONS/0106-the-bus-is-nats.md) moved the mesh's bus to NATS. Everything
+> in this document that names an exchange, a queue or a routing key — the event exchanges, the
+> durable `<node>.<module>.events` queue, the shared `serve.<key>` queue — describes the transport
+> being retired, and the conformance fixtures were captured against it.
+>
+> What does **not** change is this document's model, which is the part ADR 0074 decided: a floor
+> plus independent capabilities, an SDK that implements what it claims and is legitimate when it
+> claims less, identity taken from the sealed credential rather than the environment, at-least-once
+> with dedup on `x-event-id`, and conformance as executable fixtures per capability rather than
+> prose. The envelope keeps its shape ([ADR 0042](../../02-DECISIONS/0042-the-shape-of-an-event-on-the-wire.md));
+> it becomes the message body.
+>
+> Rewriting the wire sections onto the subjects and streams of
+> [design 25](25-the-bus-on-nats.md) §2–§3, and recapturing the fixtures there, is **step 3 of
+> [ADR 0115](../../02-DECISIONS/0115-the-bus-is-built-in-five-steps.md)**. Until that lands, read
+> the sections below for what two implementations may not disagree *about*, and design 25 for what
+> they will disagree about it *on*. A specification that silently described a retired transport
+> would be worse than an absent one, because it reads as current — hence this note rather than a
+> quiet edit.
+
 ## The shape of it
 
 A **floor** every implementation needs, and three **capabilities** that are independent of each