The mesh knows which host runs a machine, and says who is behind another

novox/hq 04-ISSUES/087. A host refuses a declaration carrying a field it
does not know, and refuses it WHOLE — deliberately, because that keeps a
half-understood declaration off a machine. It makes every new declaration
field a flag day: hosts first, then the controller. The mesh had no record
of which host any machine ran, so that order was kept by somebody
remembering it, and a machine that refused for this reason reported a
failure with nothing saying why.

The machine has reported its host version since ADR 0141. The
controller's own copy of the report did not have the field, so it was
unmarshalled into nothing and thrown away on arrival. It has it now,
records it, and shows it in `node show` — "not reported" rather than
blank, because a machine that has not said is not a machine running
nothing.

Status says which machines run an older host than another machine does,
and which is newest. Deliberately disagreement rather than staleness:
nothing delivers a host version yet (ADR 0141, accepted and not built), so
the mesh holds no canonical current version and cannot honestly say a
machine is behind THE host. What it can say is that the oldest host in the
mesh is what the mesh may send.

A machine that has reported nothing is left out rather than called
behind. Versions compare as strings, which suits the timestamps and
commits this mesh uses and is wrong for a scheme where "10" sorts before
"9" — said in the code, at the place that would have to learn.

internal/link/enrolment.go

@@ -312,6 +312,14 @@ func (e Enrolment) Heard(ctx context.Context, report Report) (news bool, err err
 			return false, err
 		}
 	}
+	// Which host produced this report (novox/hq 04-ISSUES/087), whenever it says. Recorded on every
+	// report that carries it and never cleared by one that does not — a bare word that the node is
+	// there says nothing about its host, and a machine whose host predates this reports none.
+	if report.Host != "" {
+		if err := e.Inventory.RecordHostVersion(ctx, node.ID, report.Host); err != nil {
+			return false, err
+		}
+	}
 	// What it says about the tunnel it carried (novox/hq ADR 0105), whenever it says it.
 	if report.Tunnel != nil {
 		if err := e.Inventory.RecordCarriedTunnel(ctx, node.ID, inventory.Carried{