RR-2: a kept event's stream is SEAT_<seat>_<event>

Builds: design 47 G §260, §396, §409

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

go/seatdef/seatdef.go

@@ -254,8 +254,11 @@ const (
 	KeepStream = "stream"
 )
 
-// KeptStream is the stream an event with Keep is kept in: `<seat>.<event>`, as a family is `<seat>.events`.
-func KeptStream(seat string, m Message) string { return seat + "." + m.Name }
+// KeptStream is the stream an event with Keep is kept in: `SEAT_<seat>_<event>` (design 47 G §260, §396).
+func KeptStream(seat string, m Message) string { return KeptStreamPrefix + seat + "_" + m.Name }
+
+// KeptStreamPrefix begins the name of every kept event's stream.
+const KeptStreamPrefix = "SEAT_"
 
 // legacyMode is the string form of Keep, empty for none or for the object form.
 func (m Message) legacyMode() string {

go/seatdef/seatdef_test.go

@@ -266,3 +266,10 @@ func TestAnActingRuleReadsItsStepsDefaults(t *testing.T) {
 		}
 	}
 }
+
+// Design 47 G §260, §396: a kept event's stream is SEAT_<seat>_<event>.
+func TestAKeptEventsStreamIsNamedFromTheSeatAndTheEvent(t *testing.T) {
+	if got := KeptStream("mesh-delivery", Message{Name: "owed"}); got != "SEAT_mesh-delivery_owed" {
+		t.Errorf("the stream is %s", got)
+	}
+}