test: Add a browser based test harness (#2641)

scripts/release-version-plugin.js

@@ -0,0 +1,25 @@
+import { RELEASE_VERSION_TOKEN } from './release-version.js';
+
+// What the card substitutes to mean "the version in package.json". The
+// development substitution is not used here because it appends a git hash that
+// only a build step knows.
+const PACKAGE_VERSION = 'pkg';
+
+/**
+ * Substitutes the release version the way the build does.
+ *
+ * The card reads it out of a string literal that Rollup rewrites, so without
+ * this it renders the placeholder itself: the loading screen shows the raw
+ * token, which then appears in every failure screenshot.
+ */
+export const releaseVersion = () => ({
+  name: 'release-version',
+
+  transform(code, id) {
+    if (!id.includes('src/utils/diagnostics.ts')) {
+      return null;
+    }
+
+    return { code: code.replace(RELEASE_VERSION_TOKEN, PACKAGE_VERSION), map: null };
+  },
+});