blog: add secret-zero bootstrap diagram; clean up hero images

- Add the secret-zero bootstrap-chain diagram (was a broken image ref)
- Remove the corner sparkle artifact from all six blog hero images
- Bump hero/diagram cache-bust params so the updated assets serve

src/pages/blog/[slug].astro

@@ -36,7 +36,7 @@ const { Content } = await render(entry);
       {entry.data.hero && (
         <figure class="post__hero" data-reveal>
           <img
-            src={`${entry.data.hero}?v=1`}
+            src={`${entry.data.hero}?v=2`}
             alt={entry.data.heroAlt ?? entry.data.title}
             width="1600"
             height="1073"