[fpe] check flow initialization

Strictly speaking this is not correct, since uninitialized flows are
valid flows. But for the purpose of electric we work with initialized
ones. We can remove or make this check optional in the future if need
be.

src/hyperfiddle/incseq/flow_protocol_enforcer.cljc

@@ -21,7 +21,8 @@
                   (if (first (swap-vals! !should-step? not)) (cannot-throw nm step) (violated nm "double step")))
            done (fn [] (if (first (reset-vals! !done? true)) (violated nm "done called twice") (cannot-throw nm done)))
            cancel (try (input-flow step done)
-                       (catch #?(:clj Throwable :cljs :default) e (violated "flow process creation threw" e)))]
+                       (catch #?(:clj Throwable :cljs :default) e (violated nm "flow process creation threw" e)))]
+       (when (= ::init @!should-step?) (violated nm "missing initial step"))
        (reify
          IFn (#?(:clj invoke :cljs -invoke) [_] (cannot-throw nm cancel))
          IDeref (#?(:clj deref :cljs -deref) [_]