ui: keep the board visible in light mode + bust diagram cache

Light mode: revert the heavy white-out scrim back to a moderate wash so the
board stays visible, and earn legibility with a soft light text-halo behind
running text instead (invisible on the solid cards, only works over the image).
Pairs with the darker light-mode text tokens.

Also append ?v=2 to every /diagrams/*.svg reference so the recoloured diagrams
bypass Cloudflare's 4h edge cache and show immediately (the SVG URLs aren't
content-hashed like the CSS bundle).

src/components/Diagram.astro

@@ -22,7 +22,7 @@ const cap = caption ?? captions[name];
 <figure class="diagram" data-reveal>
   <img
     class="diagram__svg"
-    src={`/diagrams/${name}.svg`}
+    src={`/diagrams/${name}.svg?v=2`}
     alt={cap ?? `${name} architecture diagram`}
     loading="lazy"
     decoding="async"