SW nav: owl logo with light/dark crossfade on .on-dark

Replace the text wordmark with the cobalt-recoloured owl + ShopWisdom single-line
lockup (Asset 46/47). The top-left logo now flips to the reverse (white) version over
dark bands. Verified locally in both modes. Staging/preview only.

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

index.html

@@ -50,7 +50,8 @@
 
 <header class="nav" id="nav">
   <a class="nav-logo" href="#top" aria-label="Shop Wisdom — home">
-    <span class="nav-wordmark">Shop<span>Wisdom</span></span>
+    <img class="nav-logo-img nav-logo-light" src="assets/logo/logo-horizontal.svg" alt="Shop Wisdom" width="214" height="36" />
+    <img class="nav-logo-img nav-logo-dark" src="assets/logo/logo-horizontal-rev.svg" alt="" aria-hidden="true" width="214" height="36" />
   </a>
   <nav class="nav-links-desktop" aria-label="Main">
     <a href="#work">The work</a>