test(browser): drive an undefined reading through the whole web path

four layers carry it: the page's envelope, the driver's unwrap, the
runner's count check and the verifier's decode. each has a unit test and
only a run proves they compose. goes red both ways, decoding an absent
value as null and dropping the envelope.

test/browser/testdata/undefined-extractor/index.html

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+<!doctype html>
+<html>
+  <body>
+    <!-- One control and one counter. The spec's other two extractors look for
+         ids that are not here, which is the shape a route-scoped extractor has
+         on every step it is off its own screen. -->
+    <div id="app">
+      <button id="go" style="width:200px;height:80px">go</button>
+      <div id="count">0</div>
+    </div>
+    <script>
+      let taps = 0;
+      document.getElementById("go").addEventListener("click", function () {
+        document.getElementById("count").textContent = String(++taps);
+      });
+    </script>
+  </body>
+</html>

test/browser/testdata/undefined-extractor/spec.ts

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+import { always, extract, taps } from "@sanderling/spec";
+
+// Two extractors that resolve nothing, the shape every route-scoped extractor
+// has on the steps it is off its own screen (folio has nine of them). JSON has
+// no undefined, so these are what the {value} envelope exists for.
+const absent = extract((s) => s.ax.find({ id: "nothing-is-ever-here" })).named("absent");
+const absentText = extract((s) => s.ax.find({ id: "missing" })?.text).named("absentText");
+
+const bumps = extract((s) => {
+  const el = s.ax.find({ id: "count" });
+  return el ? parseInt(el.text, 10) || 0 : 0;
+}).named("bumps");
+
+// A reading the page could not take must arrive as undefined, not as null and
+// not as goja's reading of the dump. This property is what a spec sees.
+const undefinedStaysUndefined = always(
+  () => absent.current === undefined && absentText.current === undefined,
+);
+
+// The run has to actually be doing something, or the property above holds for
+// the wrong reason.
+const counterNeverMoves = always(() => bumps.current === 0);
+
+export const properties = { undefinedStaysUndefined, counterNeverMoves };
+
+export const actionsRoot = taps;