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/push.go

@@ -212,6 +212,9 @@ func serve(ctx context.Context) (err error) {
 	givenEvents = bus
 	// Composed now and kept current, before the verb that answers from it is served.
 	go statusFrom.keep(ctx)
+	// The facts snapshot a merge check is fed (novox/hq to-be 45 §9): kept current while this
+	// controller holds the lease, read by the build seat from the artifact store.
+	go exportingFacts(ctx, open, bus.Conn.ConnectedServerVersion)
 	// Every call carries the lease's epoch, and its record is written only under the lease (novox/hq
 	// to-be 45 §6).
 	link.Calls.UnderLease(func() (uint64, error) { return theLease.epoch(ctx) })