site build: arc as index (CSP-clean node assembler), healthz endpoint, ship webps in-repo

nginx/default.conf

@@ -36,6 +36,12 @@ server {
         add_header Cache-Control "no-cache" always;
     }
 
+    # Probe endpoint - no file IO, no cache headers, invisible to the CDN.
+    location = /healthz {
+        access_log off;
+        return 200 "ok";
+    }
+
     # ---- Routing -------------------------------------------------------------
     location / {
         try_files $uri $uri/ $uri.html =404;