Hero loop: swap to the Adobe Stock flowing-wave clip (mp4-only)

Replace the spheres with AdobeStock_1457104053 — abstract flowing wave/particle
field on near-white (blends into paper). Optimized 60MB->1.1MB H.264 (1280x720,
crf 25, faststart). Dropped the webm: VP9 came out larger than H.264 for this
thin-line content, and H.264 plays everywhere, so a single mp4 source is smaller
and universal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

index.html

@@ -91,7 +91,7 @@
     </div>
     </div>
     <div class="hero-media" aria-hidden="true">
-      <video class="hero-video" muted loop playsinline preload="none" data-src="assets/media/hero-loop.webm" data-src-mp4="assets/media/hero-loop.mp4"></video>
+      <video class="hero-video" muted loop playsinline preload="none" data-src-mp4="assets/media/hero-loop.mp4"></video>
     </div>
   </section>