Keep a facts snapshot for merge checks, and say when it goes stale (hq to-be 45 Phase 5, S14)

Every check the mesh had was right about the world it was given and none was given
the mesh's: a real machine's name made an identity too long (263), the node-engine
refused what the catalogue check passed (236). The controller now composes what a
check needs - every machine under a pseudonym of its name's length, its roles,
system, builds, capabilities, assignments, pins, settings and how its declaration
composes; every seat, module and source; the bus, store and node-engine versions it
runs - with no secret, no address and no name, and keeps it in the artifact store
as facts:latest when it moved, or daily. The replaced snapshot's manifest is let go
of, so the nightly collector takes it. S14 raises facts-stale past two days.

cmd/mesh-controller/watchdogs.go

@@ -89,6 +89,16 @@ type signalFacts struct {
 	// lease is this controller's standing to the lease, and the epochs that ended lately (S12).
 	lease    leaseFacts
 	leaseErr error
+
+	// facts is the snapshot this controller keeps for merge checks (S14).
+	facts factsFacts
+}
+
+// factsFacts is when the newest facts snapshot was taken, when this controller began keeping it, and
+// the last attempt's error.
+type factsFacts struct {
+	taken, began time.Time
+	err          error
 }
 
 type leaseFacts struct {
@@ -313,6 +323,7 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 	f.advisories = link.Advisories.Since(now.Add(-advisoryQuiet))
 	f.lostConsumers, f.advisoriesErr = w.lostConsumers(ctx, f.advisories)
 	f.handActs, f.handActsErr = w.gatherHandActs(ctx, now)
+	f.facts.taken, _, f.facts.began, f.facts.err = exportedFacts.last()
 	return f
 }