fix(web): keep an undefined reading's index through JSON

json has no undefined, so an extractor whose getter returned one had its
whole index dropped by JSON.stringify. that index then kept goja's
dump-derived value while its neighbours held the page's, and a property
comparing previous to current across the split fires on a healthy app.
folio has nine on(route, tag) extractors, so this was most extractors on
most steps.

each reading is wrapped in a {value} envelope: the drop now happens
inside the entry, and an absent value means the getter returned
undefined, which is what the goja host records for the same getter. a
json null would instead claim it returned null and x.current ===
undefined would answer differently on the two hosts.

internal/driver/chrome/driver.go

@@ -787,16 +787,39 @@ func (d *Driver) EvaluateExtractors(ctx context.Context) (map[int]json.RawMessag
 		return nil, fmt.Errorf("decode extractor map: %w", err)
 	}
 	result := make(map[int]json.RawMessage, len(stringMap))
-	for key, value := range stringMap {
+	for key, entry := range stringMap {
 		index, err := strconv.Atoi(key)
 		if err != nil {
 			return nil, fmt.Errorf("non-integer extractor key %q", key)
 		}
-		result[index] = value
+		reading, err := extractorReading(entry)
+		if err != nil {
+			return nil, fmt.Errorf("extractor %d: %w", index, err)
+		}
+		result[index] = reading
 	}
 	return result, nil
 }
 
+// extractorReading unwraps one entry of the page's extractor table. The page
+// wraps every reading in a {"value": ...} envelope (evaluateExtractors in
+// pkg/spec/src/web-runtime.ts) because JSON has no undefined: an absent `value`
+// is the getter returning undefined, and returning it as an empty payload is
+// what makes the goja host record undefined too. Reading it as JSON null would
+// claim the getter returned null, so `x.current === undefined` would answer one
+// thing on native and another on web.
+func extractorReading(entry json.RawMessage) (json.RawMessage, error) {
+	var envelope struct {
+		Value json.RawMessage `json:"value"`
+	}
+	if err := json.Unmarshal(entry, &envelope); err != nil {
+		return nil, fmt.Errorf(
+			"reading %s is not a {\"value\"} envelope; the page and the host are "+
+				"running different bundles: %w", entry, err)
+	}
+	return envelope.Value, nil
+}
+
 // SetLastAction installs the previous step's action as state.lastAction inside
 // the page runtime. The page cannot derive it: only the runner knows which
 // action was actually applied. Without this call every web state.lastAction is