Add port labels, modernize patch panels, add a real PNG preview

- export_topology.py: edges now carry the exact port name on both ends
  (pulled directly from Cable/CableTermination, not inferred from the
  trace chain), so the diagram can label each connector.
- generate_drawio.py: patch panels are now a small rack-unit shape
  (header strip + port ticks, parented to the panel body so they move
  together) instead of a plain box; connectors show 'porta <-> portb'
  and no longer carry a directional arrowhead (these are physical
  cables, not directed links); switches keep the network-switch icon.
- render_png.js + render_host.html: headless Puppeteer driver for
  embed.diagrams.net's postMessage export protocol -- an actual
  render of the .drawio file, not an approximation. Field-Switch-
  Topology.png is the committed output, for anyone who wants a look
  without opening draw.io.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NM3nivbufxEWiKKmatEtEe

topology/render_host.html

@@ -0,0 +1,24 @@
+<!doctype html>
+<html><body style="margin:0">
+<iframe id="f" src="https://embed.diagrams.net/?proto=json&spin=1&embed=1&libraries=1&noSaveBtn=1"
+        style="width:1600px;height:1200px;border:0"></iframe>
+<script>
+window.__RESULT__ = null;
+window.__ERR__ = null;
+const iframe = document.getElementById('f');
+window.addEventListener('message', (evt) => {
+  let msg;
+  try { msg = JSON.parse(evt.data); } catch (e) { return; }
+  console.log('HOST EVENT:', msg.event);
+  if (msg.event === 'init') {
+    iframe.contentWindow.postMessage(JSON.stringify({ action: 'load', xml: window.__XML__, autosave: 0 }), '*');
+  } else if (msg.event === 'load') {
+    iframe.contentWindow.postMessage(JSON.stringify({ action: 'export', format: 'xmlpng', xml: window.__XML__, border: 24, scale: 2 }), '*');
+  } else if (msg.event === 'export') {
+    window.__RESULT__ = msg.data;
+  } else if (msg.event === 'error') {
+    window.__ERR__ = JSON.stringify(msg);
+  }
+});
+</script>
+</body></html>