The controller asks the build role that has a holder, and hears both roles' outcomes (hq ADR 0190, the handover)

A controller that asked node-build-agent from its first run would queue every build where nothing
pulls, and the build that registers build-agent — the first holder — would be among them. So the
role is chosen at ask time from the catalogue: the current role when any assigned module claims it,
the retired one while only the builder does, the current one when neither. Outcomes are followed on
both seats, the controller may publish to both, and a build's log is read under whichever role did
it; a machine on the retired role is proven on the bus to take that role's asks. The switch order
is written where the role is named, and the retired half is marked for removal with the seat row.

internal/link/builds.go

@@ -23,12 +23,21 @@ import (
 // builds, and the work shared among them (novox/hq ADR 0190). The name stays for every caller; what
 // it names moved from the mesh's one build machine to whichever build agent is idle.
 //
-// **Switching a live mesh over, in order.** The old seat's stream and worker
-// (SEAT_MESH_BUILD_MACHINE, SEAT_MESH_BUILD_MACHINE_worker) stay on the bus until removed by hand,
-// and the builder module keeps draining them while it is assigned. From the moment a controller
-// with this name runs, new asks go to node-build-agent and wait in its stream until some machine
-// holds the seat. So: let the queued builds finish; roll this controller; register and assign
-// build-agent to the machines that build; unassign builder and forget it and its seat's stream.
+// **Switching a live mesh over, in order** — and why no step strands a build. The old seat's
+// stream and worker (SEAT_MESH_BUILD_MACHINE, SEAT_MESH_BUILD_MACHINE_worker) stay on the bus until
+// removed by hand, and the builder keeps draining them while it is assigned, because a machine
+// serves the seat its credential claims (BuildSeatClaimed) and the controller asks the seat that
+// has a holder (buildSeatAmong in the command) and hears both seats' outcomes:
+//
+//  1. Merge the controller and the host's first user list together; the new controller rolls and,
+//     seeing only the builder assigned, still asks mesh-build-machine — which the builder holds.
+//  2. Merge the catalogue's build-agent; the builder builds it and the controller registers it.
+//  3. On each machine that builds: `module issue build-agent --node <n>`, then `assign`, then
+//     `push`. The first holder appears, and from then on asks go to node-build-agent.
+//  4. Unassign builder everywhere and `module forget` it.
+//  5. By hand: delete SEAT_MESH_BUILD_MACHINE and its worker, drop the retired seat row and
+//     TheBuildMachineBefore with it, and the second entries in seatsTheControllerAsks and
+//     ControllerFollows.
 const TheBuildMachine = "node-build-agent"
 
 // TheBuildMachineBefore is the role a build was submitted to until ADR 0190: the mesh's one build