fix(home): early boot reads load as an async data: script so stylesheets no longer hold them back (BUG-1127)

An inline classic script after the stylesheets waited for them to download;
on staging the reads left at 2.4-3.1 s. Measurement script gains
--css-delay and --ua iphone.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4f2nya58RoRu4yEmJgRGE

frontend/src/app/layout.tsx

@@ -5,7 +5,7 @@ import { Toaster } from "@/components/ui/sonner";
 import { StaleBuildGuard } from "@/components/stale-build-guard";
 import { StaleClientRecovery } from "@/components/stale-client-recovery";
 import { ViewportScrollLock } from "@/components/viewport-scroll-lock";
-import { bootEarlyReadsScript } from "@/lib/boot-early-reads";
+import { bootEarlyReadsScriptSrc } from "@/lib/boot-early-reads";
 import { firstPaintFallbackBootScript } from "@/lib/first-paint-fallback";
 import { themePreferenceBootScript } from "@/lib/theme-preference";
 
@@ -59,8 +59,10 @@ export default function RootLayout({ children }: Readonly<{ children: React.Reac
         {/* Classic script, not a module: it has to run when the client bundle never does. */}
         <script dangerouslySetInnerHTML={{ __html: firstPaintFallbackBootScript() }} />
         {/* Home cold-boot reads leave with the HTML instead of after the bundle
-            runs (BUG-1127). The script is a no-op on every path except "/". */}
-        <script dangerouslySetInnerHTML={{ __html: bootEarlyReadsScript() }} />
+            runs (BUG-1127). The script is a no-op on every path except "/".
+            async + data: URL, not inline: an inline script after the
+            stylesheets would wait for them to download (fix 2026-10-01). */}
+        <script async src={bootEarlyReadsScriptSrc()} />
         {enableReactDevTools && (
           <>
             <Script

frontend/src/lib/boot-early-reads.ts

@@ -78,6 +78,18 @@ export function bootEarlyReadsScript(): string {
 })();`;
 }
 
+/**
+ * The script as an async data: URL (BUG-1127 fix, 2026-10-01). An inline
+ * classic script placed after the page's stylesheets waits until they have
+ * downloaded (script-blocking stylesheets), which on staging held the early
+ * reads back 2.4-3.1 s. An `async` external classic script is not blocked by
+ * stylesheets, and a data: URL costs no round trip. Plain ASCII base64, so
+ * the attribute needs no escaping.
+ */
+export function bootEarlyReadsScriptSrc(): string {
+  return `data:text/javascript;base64,${Buffer.from(bootEarlyReadsScript(), "utf8").toString("base64")}`;
+}
+
 function bootReads(): BootReads | null {
   if (typeof window === "undefined") return null;
   const reads = (window as unknown as Record<string, BootReads | undefined>)[BOOT_READS_GLOBAL];