feat(05-01): add Paginate envelope and RegisterJoinTable

- Page/PageMeta emit {data, meta} with no links key
- RegisterJoinTable fails loud on nil db; document pivot-write contract

lagoon/relations_test.go

@@ -0,0 +1,16 @@
+package lagoon
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRegisterJoinTableNilDB(t *testing.T) {
+	err := RegisterJoinTable(nil, struct{}{}, "Artists", struct{}{})
+	if err == nil {
+		t.Fatal("want non-nil error")
+	}
+	if !strings.Contains(err.Error(), "nil") {
+		t.Fatalf("got %v", err)
+	}
+}