feat(events): causal timeline over recorded links only

A bounded walk (six hops, two hundred events) over the links events
actually record: causation ids, parent ids and cited-event evidence,
backwards towards causes and forwards towards consequences, with the edge
kind on every arrow. Nodes carry their cycle for grouping, labelled as
correlation rather than causation, and the artefacts (beliefs, memories,
model calls) they rest on. Cyclic references terminate. Exposed at
GET /api/v1/events/{id}/chain and /events/{id}/timeline, linked from the
event page, and documented.

tests/m1/m1_test.go

@@ -229,6 +229,14 @@ func TestFirstMilestone(t *testing.T) {
 	if len(detail.Assertions) != 1 || detail.Assertions[0].AttributedTo != "admin" || detail.Assertions[0].SourceEvent != ids.ID(detail.Evidence[0].RootIDs[0]) {
 		t.Fatalf("assertion: %+v", detail.Assertions)
 	}
+	// The causal chain walks recorded links only: the message leads to the belief.
+	var chain struct {
+		Nodes []struct{ Depth int }
+		Edges []struct{ Kind string }
+	}
+	if code := h.do("GET", "/api/v1/events/"+detail.Evidence[0].RootIDs[0]+"/chain?direction=forward&depth=4", nil, &chain); code != 200 || len(chain.Nodes) < 2 || len(chain.Edges) == 0 {
+		t.Fatalf("chain: %d %+v", code, chain)
+	}
 	var evCtx api.EventContext
 	h.do("GET", "/api/v1/events/"+detail.Evidence[0].RootIDs[0]+"/context", nil, &evCtx)
 	if evCtx.Event.Type != events.Observation || len(evCtx.Children) == 0 {