Merge pull request 'Design 28: 5.5 done, the mesh has one bus; issue 131 resolved' (#156) from design/28-one-bus-issue-131-resolved into main

The AMQP transport is gone from the control plane and the hosts (mesh-controller #112,
mesh-host #39). On the way: no build had ever recorded its bases, so every bases-first order
walked an empty graph; the builder now reports what it was handed and the graph is read from
builds (mesh-controller #113/#114). Issue 131 is resolved by the forge module's merge event,
the control plane following it, and those edges.

03-DESIGN/01-to-be/28-building-the-bus.md

@@ -5,7 +5,7 @@ code:
   - mesh-catalog modules/nats
   - mesh-controller internal/catalogue
   - mesh-lab scenarios
-updated: 2026-09-27
+updated: 2026-09-28
 decisions:
   - 02-DECISIONS/0116-the-bus-is-built-in-five-steps.md
   - 02-DECISIONS/0106-the-bus-is-nats.md
@@ -694,8 +694,27 @@ healthy while reacting to nothing.
       > shutting it down ends the path that reaches this installation's machines from a workstation.
       > The rollout is driven from the node, or before the broker stops — a sequencing constraint on
       > 5.2, not an afterthought.
-- [ ] 5.5 **the AMQP transport is deleted from the control plane and the hosts**, and the variable
-      that selected a transport is refused at start as unknown. One bus, nothing to select ([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)).
+- [x] 5.5 **the AMQP transport is deleted from the control plane and the hosts**. One bus, nothing
+      to select ([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)).
+
+      **Done 2026-09-28.** The control plane's old consume loop, build request, tool call, management
+      API and account scoping went, and the host's old dialling and enrolment paths with them; a
+      membership or token naming any other bus is refused before anything is sent. Nothing selects a
+      transport any more: the variable that once did (`MESH_BUS_NATS`) now only names where the
+      control plane reads its own bus credential, the way any module reads a secret. **Checked by the
+      build**: neither repository's module file names the AMQP client library, so a line that still
+      used it would not compile. The store-window guarantee ([issue 083](../../04-ISSUES/083-other-control-messages-are-lost-while-the-store-restarts/00-report.md))
+      is tested against a bus-less fake rather than the old transport's memory, which is what let
+      that memory go — the one thing it did that the stream does not (superseding a held report) is
+      the staleness check on the message itself (design 25 §3).
+
+      Found on the way: **no build had ever recorded what it stood on.** A recipe reads its base from
+      a build argument, so the digest was never in the file the builder derived edges from, and every
+      order that says *bases first* — `build --on`, `build --behind`, the merge follow-up of
+      [issue 131](../../04-ISSUES/131-nothing-tells-the-mesh-a-source-moved/00-report.md) — walked a
+      graph with no edges. The builder now reports the bases it was handed, the control plane records
+      them by artifact path, and the graph is read from the newest build of each module — a recorded
+      manifest carries no `build.on`, so the edge is derived from the build or it does not exist.
 
 > **The old 5.4 note is history.** It recorded that a retirement *condition* was wrong from
 > [ADR 0127](../../02-DECISIONS/0127-amqp-is-a-provision-not-the-bus.md) onward, which framed the old broker

04-ISSUES/131-nothing-tells-the-mesh-a-source-moved/00-report.md

@@ -0,0 +1,107 @@
+---
+status: resolved
+opened: 2026-09-27
+located-in: [mesh-catalog modules/gitea, mesh-controller cmd/mesh-controller, mesh-controller internal/builder]
+fixed-by: mesh-catalog #124 — the forge module watches for merged pull requests and emits `pull.merged` with the merge commit and the clone address; mesh-controller #110/#111 — the control plane follows that event on the bus, marks every module built from that repository and branch as moved, and builds them bases first, stopping when a base fails; mesh-controller #113/#114 — a build records the bases it was handed and the graph is read from builds, without which "bases first" had no edges to order by.
+amended-design: 03-DESIGN/01-to-be/28-building-the-bus.md
+---
+
+# 131 — Nothing tells the mesh a source moved, and it reports itself current anyway
+
+## What was observed
+
+Six changes were merged to the trunk of six repositories in one sitting. The build machine built
+nothing. Its last build, minutes before the first merge, was still the one it reported; no build was
+requested, refused or failed, because none was ever asked for.
+
+Asked afterwards what was wrong, the mesh said:
+
+> 4 machine(s), all doing what they were told, all heard from, running what the mesh would send them,
+> and every module current with its source
+
+Every one of the six had moved. The last clause was false for all of them, and it is the clause a
+person reads to decide whether there is anything to do.
+
+## Why it matters beyond this instance
+
+**The mesh learns a source moved by being told, and there is no longer anything to tell it.** The
+command exists — a person names the module and the commit — and so does the question the overview
+answers. What is missing is whatever used to connect the two. One repository still carries a forge
+webhook aimed at a port; the rest carry none, and the port belongs to a different service than the
+one the arrangement implies. So the state is not "the trigger is broken" but "there is no trigger,
+and nothing says so".
+
+**A wrong answer is worse here than no answer.** "Every module current with its source" is
+indistinguishable, to a reader, from a mesh that has genuinely caught up. The overview is built to be
+the thing you check instead of checking by hand, so a confident false negative removes the habit that
+would otherwise have caught it. Nothing in the mesh is at fault for being out of date — it is at
+fault for saying it is not.
+
+**It is also why "current with its source" cannot be a stored fact.** The mesh compares what it built
+against what it was last told the source was, and calls that agreement. Two facts agreeing tells you
+nothing when both come from the same place.
+
+## The intended shape, which is decided and not built
+
+The forge emits what happened to it — a pull request merged — and the build machine reacts by
+building what that commit affects. That keeps the forge ignorant of the build system and the build
+machine ignorant of the forge's internals, which is the same argument
+[ADR 0126](../../02-DECISIONS/0126-a-module-declares-its-own-seats.md) makes for addressing an event
+to its emitter: the merge is a fact about the forge, and what should be rebuilt because of it is not
+the forge's business to know.
+
+The forge's module already declares the event. The build machine declares that it consumes nothing.
+
+## What the trigger cannot be
+
+**Not one build per changed module.** The modules form a graph: several are built from one
+repository, and some are the base another is compiled on — a runtime image, a compiler base, a
+repository whose context a second module builds from. Firing a build for each changed module
+independently would start work that cannot succeed yet and produce a failure per dependent, for one
+cause.
+
+Observed while catching the mesh up by hand on 2026-09-27: a compiler base had to move before
+anything compiled against it could build, and when it failed, the right behaviour was for its
+dependents to wait rather than each fail the same way. Fifteen modules shared the cause. A trigger
+that reports it fifteen times has buried it.
+
+So whatever reacts to the forge's event resolves what changed into an order, builds the bases first,
+and holds a dependent while its base is unbuilt or failed. That is a larger thing than "rebuild what
+the commit touched", and knowing it now is cheaper than discovering it from fifteen identical
+failures.
+
+## Open questions
+
+- Is "the source moved" still a thing a person can assert by hand once the event path exists, or does
+  the hand-operated form become the thing that made this failure possible?
+- Which commit does the build machine act on — the merge, or each commit it brought — and what does
+  it do when several arrive for one module at once?
+- How does the overview stop being able to lie? Comparing what was built against what was recorded
+  will always agree. Whether the trunk has moved is a question only the forge can answer, so either
+  the overview asks it, or it stops claiming to know.
+- Does this want to be the same mechanism as the build request on the bus
+  ([ADR 0129](../../02-DECISIONS/0129-a-seat-carries-the-protocol-of-its-role.md)), or does it sit in
+  front of it?
+
+## What was done (2026-09-28)
+
+The shape above was built as described: the forge's module emits the merge, the control plane
+consumes it, and nothing on either side knows the other's internals. The build is asked for the
+merge commit, not each commit the merge brought — the trunk moved once, to one place. Several merges
+for one module arriving in a row are followed in turn, each moving the recorded source to its own
+commit, so the last one to arrive is the one the mesh ends up built from.
+
+The hand-operated form stays. `module moved` is how a source is recorded without a forge — a module
+built from a repository elsewhere, or a mesh whose forge module is down — and it is the same act the
+event performs, so the two cannot disagree about what "moved" means.
+
+**Bases first needed edges, and there were none.** The order this report asked for was written and
+walked a graph that no build had ever recorded: a recipe reads its base from a build argument, so the
+digest was never in the file the builder read edges from. A build now reports what it was handed, the
+control plane records it by artifact path, and the order is read from each module's newest build.
+
+**What still can lie.** The overview compares what was built against where it was last told the
+source is; the forge's event is now what moves that mark, so it is right for as long as the forge
+module was listening. A merge made while that module was down is a merge the mesh does not know of
+until the module next polls — it announces what merged since it last looked, so the gap closes when
+it comes back, and not before. The overview does not say so.