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/signals_test.go

@@ -37,6 +37,7 @@ func calm(now time.Time) *signalFacts {
 		selfCheck:     selfCheckFacts{last: now.Add(-time.Minute), every: 5 * time.Minute},
 		lostConsumers: map[string]bool{}, epochs: map[int64]inventory.Epoch{},
 		lease: leaseFacts{held: true, epoch: 57, renewed: now.Add(-2 * time.Second)},
+		facts: factsFacts{taken: now.Add(-time.Hour), began: now.Add(-time.Hour)},
 	}
 }
 
@@ -125,6 +126,11 @@ var suppressions = map[string]suppression{
 		inside: func(f *signalFacts) { f.staleRefusals = refusedBy(f.now, 41, 5) },
 		past:   func(f *signalFacts) { f.staleRefusals = refusedBy(f.now, 41, 6) },
 	},
+	// The snapshot a merge check is fed, older than two days; or none kept by a controller two days up.
+	"S14": {
+		inside: func(f *signalFacts) { f.facts.taken = f.now.Add(-47 * time.Hour) },
+		past:   func(f *signalFacts) { f.facts.taken = f.now.Add(-49 * time.Hour) },
+	},
 	// Twice by hand within a fortnight is a healer wanted; once, or the first of two a day too old, is not.
 	"S15": {
 		inside: func(f *signalFacts) {