Add signed updates from a bundle or a URL

router-release signs a release as a binary cache; router-update verifies
it against the router's update keys, migrates settings.json, and applies
it with commit-confirm. Includes the web UI page and a VM test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/templates/layout.html

@@ -14,6 +14,7 @@
   <nav>
     <a href="/">Overview</a>
     <a href="/network">Network</a>
+    <a href="/update">Update</a>
     <a href="/advanced">Advanced</a>
     <form method="post" action="/logout"><input type="hidden" name="csrf" value="{{.CSRF}}"><button class="link">Log out</button></form>
   </nav>