A build machine serves the seat its credential claims (hq ADR 0190, the handover)

After the build role moved to node-build-agent, nothing would hold it until build-agent is
registered — and registering build-agent needs a build outcome that only the running builder
could produce, bound as it was to the old seat by name. One binary, two roles: the seat a machine
serves is the first its credential claims, as the mesh writes the claims beside the credential it
issues (ADR 0159); the old builder keeps draining mesh-build-machine, a build-agent takes
node-build-agent, and what each says about a build goes out as that seat's events, so an outcome
is heard where the asker of that seat listens. A credential naming no claim serves the current role.

cmd/mesh-builder/main.go

@@ -137,7 +137,12 @@ func takeWorkFrom(credential Credential, on string) (link.BuildMachine, error) {
 	if err != nil {
 		return nil, err
 	}
-	return link.MachineOverNATS(js, on), nil
+	// **The seat this machine serves is the one its credential claims** (novox/hq ADR 0190, the
+	// handover): the mesh issues a build machine's credential naming the seat its module claims,
+	// and one binary serves the old role as `builder` and the new as `build-agent` from that alone.
+	seat := link.BuildSeatClaimed(credential.seatsClaimed())
+	fmt.Fprintf(os.Stderr, "taking build work as a holder of %s\n", seat)
+	return link.MachineOverNATSOn(js, on, seat), nil
 }
 
 // answer does one build and says what happened, whichever way it went.
@@ -453,6 +458,21 @@ type Credential struct {
 	// as two fields and this machine joins them once, here, to dial.
 	User     string `json:"user,omitempty"`
 	Password string `json:"password,omitempty"`
+	// Claims are the seats the module this credential was issued for claims, as the mesh writes
+	// them beside the credential (novox/hq ADR 0159). The first is the build role this machine
+	// serves; a credential naming none is from before claims travelled in it.
+	Claims []struct {
+		Seat string `json:"seat"`
+	} `json:"claims,omitempty"`
+}
+
+// seatsClaimed is the seats the credential names, in order.
+func (c Credential) seatsClaimed() []string {
+	out := make([]string, 0, len(c.Claims))
+	for _, claim := range c.Claims {
+		out = append(out, claim.Seat)
+	}
+	return out
 }
 
 // onTheNewBus is whether a credential is for the bus being built: its address says so, and the