Say in every declaration the assignment generation it came from, and record every send (hq issue 234)

A declaration newer in sequence than every other named four fewer modules
than the assignments held, a machine applied it, and nothing the mesh kept
said who sent it or from what view. The store now raises an assignment
generation in the same transaction as every assignment change (a trigger,
so a cascade counts too); a send reads it before composing, carries it to
engines that said they read it, marks a gate's put-back, and is recorded
with its sequence, sender, generation and modules. The would-send is
stamped with what was last sent, so nothing reads behind for it; a refusal
is kept on the send it refused and raised as S20 naming the sender; plan
says what the machine was last told.

cmd/mesh-controller/signals_test.go

@@ -178,6 +178,14 @@ var suppressions = map[string]suppression{
 					commit: "c0ffee001122", modules: []string{"app"}, since: f.now.Add(-time.Second)}}}
 		},
 	},
+	// A send refused by its machine for the generation it was composed from (novox/hq issue 234): said at
+	// once, naming its sender, for an hour after. Inside: the last such refusal was more than an hour ago.
+	"S20": {
+		inside: func(f *signalFacts) {
+			f.refusedSends = []inventory.Send{refusedSend(f.now.Add(-61 * time.Minute))}
+		},
+		past: func(f *signalFacts) { f.refusedSends = []inventory.Send{refusedSend(f.now.Add(-time.Second))} },
+	},
 	// 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) {