projects: add The Mirror pull-through registry case study; wire cover art for both learn-site projects

Adds mirror-registry as a featured project (order 26, beside The Exploded
Cluster) covering the zot deployment, the bootstrap and auth circular
dependencies, and the retention/revalidation behaviours that corrected the
first published write-up. Adds the cover: field to exploded-cluster (it had
none, so the card rendered imageless) and stages public/covers/image.md with
the generation prompts and processing steps for both covers.

src/content/projects/exploded-cluster.md

@@ -4,9 +4,10 @@ outcome: "A scroll-driven teaching site that takes container platforms apart one
 summary: "A single-page course on how Kubernetes and OpenShift delivery actually works - each subject drawn as one exploded illustration, sliced into its real components and wired to the scroll, then shipped through the same GitOps and CSP discipline as the rest of the estate."
 role: "Author / Engineer"
 period: "2026"
-stack: ["Astro-free static build", "GSAP ScrollTrigger", "Strict CSP", "Cloudflare Tunnel", "Argo CD", "zot mirror"]
+stack: ["Dependency-free static build", "GSAP ScrollTrigger", "Strict CSP", "Cloudflare Tunnel", "Argo CD", "zot mirror"]
 featured: true
 order: 25
+cover: "exploded-cluster"
 links:
   - label: "learn.bztmon.com"
     href: "https://learn.bztmon.com"