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/components/PostList.astro

@@ -16,7 +16,7 @@ const { posts } = Astro.props;
         <a class="post__link" href={`/blog/${post.id}/`}>
           {post.data.hero && (
             <div class="post__thumb">
-              <img src={`${post.data.hero}?v=1`} alt="" width="1600" height="1073" loading="lazy" decoding="async" />
+              <img src={`${post.data.hero}?v=2`} alt="" width="1600" height="1073" loading="lazy" decoding="async" />
             </div>
           )}
           <div class="post__card-body">