ui: Motion-driven animation layer (motion.dev)

Unify scroll reveals, hero entrance, nav active-underline, button springs,
the scroll-progress bar and backdrop parallax under one Motion system
(src/scripts/anim.ts), bundled into a self-hosted /_astro module so CSP
script-src 'self' holds. Progress bar + parallax now scroll()-driven and
cross-browser (was Chromium-only animation-timeline). Fail-open, reduced-motion
aware, content fully visible with JS off.

src/layouts/Layout.astro

@@ -70,5 +70,11 @@ const ogImage = new URL(site.ogImage, site.url).href;
       <slot />
     </main>
     <Footer />
+
+    <!-- Motion (motion.dev) animation layer. NOT is:inline → Astro/Vite bundles
+         it to a self-hosted /_astro/[hash].js (has src=) → strict CSP holds. -->
+    <script>
+      import "../scripts/anim.ts";
+    </script>
   </body>
 </html>