Caught up is an equality, not an ordering

The report carries the digest of the declaration it applied (mesh-host
8211d8b), and the mesh stores it beside the outcome. `reported` rows in
the status JSON now say `current`: whether the machine's last word
names the declaration last sent.

Not derivable from the timestamps beside it, which is why they were
not enough: an apply begun under the previous declaration reports
after the next send — newer, and still about the old words. The lab
lost exactly that race between one test's closing push and the next
test's opening one.

Empty digests — every host from before reports carried one — read as
not current, which errs toward waiting rather than toward asserting on
files that are not there yet.

cmd/mesh-control/readable.go

@@ -64,6 +64,10 @@ type machineReported struct {
 	Outcome string     `json:"outcome"`
 	At      *time.Time `json:"at,omitempty"`
 	Sent    *time.Time `json:"sent,omitempty"`
+	// Current is whether the last report names the declaration last sent. Not derivable from
+	// the timestamps beside it: an apply begun under the previous declaration reports after the
+	// next send, newer and still about the old words.
+	Current bool `json:"current"`
 }
 
 type machineWaiting struct {
@@ -98,7 +102,7 @@ func statusAsJSON(wrong []inventory.Doing, nodes []inventory.Node, quiet []inven
 		Reported: []machineReported{}}
 	for _, r := range reported {
 		out.Reported = append(out.Reported, machineReported{
-			Node: r.Node, Outcome: r.Outcome, At: r.At, Sent: r.Sent})
+			Node: r.Node, Outcome: r.Outcome, At: r.At, Sent: r.Sent, Current: r.Current})
 	}
 
 	for _, m := range waiting {