External embed: ship a minimal transparency baseline page

src/transparency-iframe.html is a tiny html+body=transparent
stub we serve at /transparency-iframe.html so dev hosts (and
future debug tools) can include it via iframe to confirm that
cross-origin iframe transparency is working in the target
browser without involving any of the Artstr embed JS.

Validated end-to-end:
- srcdoc iframe → magenta shows through ✓
- cross-origin minimal page → magenta shows through ✓
- cross-origin Artstr embed → magenta shows through the
  Computer_Icon's transparent ring ✓

What looked like opaque white on the stress-test host was the
artwork's natural fill + the host's pink/yellow checker reading
as noise rather than see-through. Magenta on a flat backdrop
made the transparency unambiguous.

src/transparency-iframe.html

@@ -0,0 +1,18 @@
+<!doctype html>
+<html>
+<head>
+<title>Minimal transparent page</title>
+<style>
+  html, body {
+    margin: 0; padding: 0;
+    background: transparent !important;
+    font: 14px system-ui, sans-serif;
+    color: #000;
+  }
+  div { padding: 8px; }
+</style>
+</head>
+<body>
+<div>Minimal page on localhost:8000.<br>If the parent's magenta backdrop shows through, cross-origin iframe transparency works in this browser.</div>
+</body>
+</html>