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.

internal/link/protocol.go

@@ -94,6 +94,9 @@ type Report struct {
 	//
 	// The node states and this context writes, which is the shape of every message here.
 	Carried []int `json:"carried,omitempty"`
+	// Declared is the digest of the declaration this report is about — the same bytes, hashed
+	// the same way, as the `sent` digest the mesh recorded. Which declaration, not when.
+	Declared string `json:"declared,omitempty"`
 }
 
 // EnrolReply is what the mesh says back.