Nav transparent until scroll; mobile hero loop rotated 90deg CW + fit

- Nav: fully transparent over the hero (no bg/blur/border), frosts in once
  scrolled past 24px (.scrolled toggled in updateNavTheme; on-dark still wins
  over dark sections).
- Mobile hero loop: rotate 90deg CW and size (240px x 100vw, centered) so it
  fills the landscape strip after rotation; warm tint ::after still applies.

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

index.html

@@ -283,6 +283,7 @@
     return false;
   }
   function updateNavTheme() {
+    nav.classList.toggle('scrolled', window.scrollY > 24);
     var nr = nav.getBoundingClientRect();
     nav.classList.toggle('on-dark', darkBehind((nr.top + nr.bottom) / 2));
     if (menu && menu.classList.contains('open')) {