Design 28: 5.3 is built and was used for the hand-over

Tasks 4.3, 5.2 and 5.4 are done as of 2026-09-28 02:25: every machine reports on
the new bus, the seat is held by the module that provides it, the old broker is
unassigned and forgotten, and every credential was minted afresh at the end.

5.2 records what it took, in the order it was found and each fixed on the trunk
before the next step, and how the bootstrap loop was broken once, by hand.

03-DESIGN/01-to-be/26-the-seats.md

@@ -4,12 +4,15 @@ status: implemented
 code:
   - mesh-controller internal/catalogue/seats.go
   - mesh-controller internal/catalogue/resolve.go
+  - mesh-controller internal/inventory/seats.go
+  - mesh-controller internal/inventory/migrations/0039-a-seat-is-held-by-one-assignment-on-record.sql
   - mesh-controller cmd/mesh-controller/seats.go
   - mesh-controller cmd/mesh-controller/source.go
   - mesh-controller internal/inventory/migrations/0032-a-source-may-live-on-a-seat.sql
   - mesh-catalog modules/gitea/module.json
 updated: 2026-09-27
 decisions:
+  - 02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md
   - 02-DECISIONS/0126-a-module-declares-its-own-seats.md
   - 02-DECISIONS/0128-the-mesh-bus-is-required-not-ambient.md
   - 02-DECISIONS/0111-a-build-source-is-on-the-git-seat-or-external.md
@@ -42,6 +45,31 @@ is refused. A seat makes a role singular, never a module.
 **The seat points at the assignment.** Everything the mesh knows about the holder is what it knows
 about that assignment: the node, the node's settings for the module, and what the module serves.
 
+**Which assignment holds a seat is a fact on record, and changes as one act.** Revision, 2026-09-27
+([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)). Until
+then the holder was derived — the module that is assigned and claims the seat holds it, and a second
+eligible assignment was refused. That has no way to pass a seat from one holder to the next without a
+moment in which nobody holds it, and the controller finds its own bus through one of these seats: that
+moment took the control plane down for an evening. So the holder is now one row the controller keeps,
+written by a handover — `seat <name> --to <node>/<module>` — that names the seat and the assignment
+taking it over and replaces the previous holder in the same write. Between two handovers the seat has
+exactly one holder, and it is never none.
+
+Three consequences follow. **A seat with no row is held as it always was**: the sole eligible
+assignment holds it, and two eligible ones are refused — so a mesh that has never handed a seat over
+behaves exactly as before, and the row appears the first time somebody does. **With a row, any other
+assignment whose module could hold the seat is eligible and silent**: neither refused nor holding.
+That is what lets the next holder run beside the current one until the handover, which the bus's move
+needs ([28](28-building-the-bus.md), task 5.3). **And a holding is the assignment's**: unassigning the
+holder takes the row with it, so a seat never points at something that is not running anywhere, and
+the seat falls back to derivation rather than to nothing.
+
+The handover refuses what would make the new holder wrong before anything is written: the seat must
+exist, the assignment must exist, and the module must be able to hold the seat — claim it at its scope
+and provide what it delivers, judged against the store's row and not against anything compiled into a
+binary. It does not check that the module is running yet; `push` confirms that afterwards, and a
+handover that could only be recorded after the new holder was up could not be the switch.
+
 **The set is closed.** A seat the mesh does not define is refused wherever it is named, and so is one
 named at the wrong scope. Adding a seat is a decision, recorded, for the reason every addition to the
 host's vocabulary is one: the set is what a person reads to learn what a mesh can have, and an entry
@@ -207,7 +235,9 @@ checked as their tables say:
 | `mesh-*` is the mesh's, and a module may not declare one | 0118: a registration test refusing a manifest that declares any `mesh-*` seat, naming the prefix. |
 | Two modules cannot declare the same seat | 0118: a registration test; the second is refused and the first untouched. |
 | A holder satisfies the seat's protocol | 0118: a claim whose module does not serve what the seat declares is refused at assignment. |
-| A seat is held by one assignment, and only by one whose module can hold it | 0118: resolution tests for a second holder and for a seat the definition does not name. |
+| A seat is held by one assignment, and only by one whose module can hold it | 0118: resolution tests for a second holder and for a seat the definition does not name. 0131: `CanHold` is the one judgement, shared by registration and the handover, and its test follows the store's row. |
+| A holder on record settles the seat; another eligible assignment is silent, not refused | 0131: resolution tests with a recorded holder on the same machine, on another machine, and under a seat's former name; without a record, the old rule's tests still pass unchanged. |
+| A handover replaces the holder as one write, needs an assignment to point at, and goes with it | 0131: store tests — a second handover leaves one row; a handover to a module not assigned where named is refused; unassigning the holder removes the row. |
 | A requirement naming a seat is answered by its holder; a foundation seat cannot be named | 0118: resolution tests with a second provider on the consumer's node, with the seat unheld, and naming `mesh-store`. |
 | Several providers and none local is a person's choice | 0118: an assignment test listing candidates with the seat's holder first and recording the pin. |
 | `secret` is reserved | 0118: the parser and resolution refusals for another provider and a pin. |

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

@@ -535,7 +535,7 @@ it, and the beds that need a mesh living on NATS can finally run.
       The outcome carries the module name, because only the manifest says what was built and one
       message now has three readers. A failed build names none: it produced no module version, and
       the catalogue would otherwise place something that was never made.
-- [~] 4.3 an installation completes over the bus, with the same outcome as the path it replaces —
+- [x] 4.3 an installation completes over the bus, with the same outcome as the path it replaces —
       **the installer can raise it**: a foundation template that stands up the server, writes the
       server's own settings and the mesh's first user list beside them, and starts a controller
       reaching the new bus. What remains is running it, which is 4.1's bed.
@@ -655,42 +655,35 @@ healthy while reacting to nothing.
 - [ ] 5.1 the cutover bed: a mesh on AMQP with a predecessor stand-in on the deprecated broker
       moves its bus in one rollout, every node reporting on NATS afterwards, the stand-in's own
       client still connected throughout
-- [~] 5.2 the rollout: accounts composed, then the controller, every host and every runtime
+- [x] 5.2 the rollout: accounts composed, then the controller, every host and every runtime
       together; every node confirmed heard before AMQP stops.
 
-      **The readiness half is in and is the half worth having.** The move takes every node at once, so
-      there is nothing to inspect afterwards and no half to roll back — either the mesh was ready or it
-      was not. `rollout check` answers that from records, with one dial: is a bus answering, does a
-      machine hold the seat, has it been sent the composed user list, does every machine and every
-      module that speaks have a credential. Each missing thing names its own next step, because "not
-      ready" that cannot be acted on is not an answer at the point where the next step is irreversible.
+      **Done 2026-09-28, 02:25.** Every machine reports on the new bus, the seat is held by the
+      module that provides it, the old broker is unassigned and forgotten, and every credential was
+      minted afresh at the end because two had been printed on the way. What it took, in the order
+      it was found, each fixed on the trunk before the next step: the control plane's `serve` and
+      `push` never selected the new transport (task 4.3, open until then); a machine's user was
+      granted neither the asking nor the delivery of its own consumer; the account had no JetStream
+      of its own; the control plane's client verified the bus's certificate by name instead of
+      pinning it; the seat table's rows carried no protocol, so no role's work queue was raised; the
+      build machine decided its bus from a variable its container never received; and a rotation
+      put new hashes on the bus before three machines had received their new memberships — which
+      is why there is now `rollout hand <node>` and a host adopts a delivered membership at start.
+      The bootstrap loop — a bus that can only be raised by a declaration that can only arrive
+      over that bus — was broken once, by hand: the mesh's own composed configuration started the
+      server, and the controller binary was run on the node directly until the managed container
+      could be rebuilt over the bus it was on.
 
-      **A machine with no credential is what must stop it.** It keeps running, cannot come back, and
-      afterwards there is no bus to tell it anything over.
-
-      The move itself is deliberately not written yet, and the command says so rather than pretending:
-      it waits on the check having been run against a real mesh. Writing the irreversible half before
-      the question it depends on has ever been asked of something real is how the plan's own rule about
-      beds gets broken by another route.
-
-      > **What this costs if it goes wrong, measured rather than assumed.** Nothing in a served
-      > request's path goes over the mesh's own bus: modules serve from their own containers. What a
-      > failed move costs is the mesh's ability to *change* anything — pushes, tool calls, new
-      > provisioning — until it is finished or undone. That is worth knowing before rather than
-      > after, and it is why the operator's "as long as my services keep running" is a reasonable
-      > position rather than a gamble. **Measured on 2026-09-27**, when a seat emptied itself
-      > mid-change: 52 containers stayed up and the broker never stopped; the control plane
-      > crash-looped for two hours and nothing could be deployed until it was repaired by hand.
-      > An earlier version of this note said the old broker stays as an ordinary provider of
-      > `amqp` ([ADR 0127](../../02-DECISIONS/0127-amqp-is-a-provision-not-the-bus.md)); that is withdrawn by [ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md) — see 5.4.
-- [ ] 5.3 **the seat changes hands as one act.** A command takes a seat and the assignment taking it
+- [x] 5.3 **the seat changes hands as one act.** A command takes a seat and the assignment taking it
       over, and the seat is never empty in between — the emptiness is the outage of 2026-09-27, when
       the control plane, which finds its own bus through this seat, lost the address and looped.
-      Today only `seat rename` exists. This is what 5.2 uses to move `mesh-broker` from the old
+      **Built 2026-09-27** (`seat_holding`, migration 0039; design 26 says how it is checked), and used
+      live the next night to hand `mesh-broker` from the old broker's assignment to the new one's. This
+      is what 5.2 uses to move `mesh-broker` from the old
       broker's assignment to the new one's, and it is built first ([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md)).
-- [ ] 5.4 **the old broker and everything that named AMQP leave the mesh** ([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md),
+- [x] 5.4 **the old broker and everything that named AMQP leave the mesh** ([ADR 0131](../../02-DECISIONS/0131-everything-on-the-mesh-speaks-to-the-broker-seat.md),
       superseding [ADR 0127](../../02-DECISIONS/0127-amqp-is-a-provision-not-the-bus.md)): the two modules that
-      required `amqp` are removed, the broker's module is unassigned and removed, registration refuses
+      required `amqp` are removed, the broker's module is unassigned and removed (**done 2026-09-28**; the predecessor's own tooling, which rode the same adopted broker, went dark with it, as [ADR 0130](../../02-DECISIONS/0130-the-predecessor-is-ending-and-its-broker-goes-with-it.md) accepted), registration refuses
       a manifest that provides or requires `amqp`, and a whole-catalogue check asserts none does. Not
       a retirement condition — a decision, taken, with the operator's "I don't care if the predecessor
       breaks" on record ([ADR 0130](../../02-DECISIONS/0130-the-predecessor-is-ending-and-its-broker-goes-with-it.md)).