mavweb: add rule trace page at /trace

New template showing the most recent tick's rule evaluation results
with color-coded table, expandable gate detail panels, nav links
to dash and history. Uses the TickTrace IPC method from #15.

cmd/mavweb/dash.html

@@ -10,7 +10,7 @@
  .updated{color:#666;font-size:.85rem;margin-top:-.5rem;margin-bottom:1rem}
  nav a{color:#8cf;margin-right:1rem;font-size:.85rem}
 </style>
-<nav><a href=/history>history</a></nav>
+<nav><a href=/history>history</a> <a href=/trace>trace</a></nav>
 <h2>presence</h2>
 <p><span class={{.Presence.Bucket}}>{{.Presence.Bucket}}</span> — score {{printf "%.2f" .Presence.Score}} ({{ago .Presence.Updated}})</p>
 <div class=updated>обновляется каждые 10с</div>