Add graph explorer UI shell

tests/test_graph_explorer.py

@@ -177,11 +177,19 @@ def test_registry_serves_graph_explorer_exports(tmp_path: Path) -> None:
             manifest = json.loads(response.read())
         with urllib.request.urlopen(f"{base_url}/exports/graph-explorer", timeout=5) as response:
             payload = json.loads(response.read())
+        with urllib.request.urlopen(f"{base_url}/ui/graph-explorer", timeout=5) as response:
+            page = response.read().decode("utf-8")
+            content_type = response.headers["Content-Type"]
 
         _validate_schema("graph-explorer-manifest.schema.yaml", manifest)
         _validate_schema("graph-explorer-payload.schema.yaml", payload)
         assert manifest["id"] == "railiance-fabric.registry-map"
         assert payload["metrics"]["registered_only_repo_count"] == 1
+        assert content_type.startswith("text/html")
+        assert 'id="graph-canvas"' in page
+        assert "cytoscape.min.js" in page
+        assert "/exports/graph-explorer/manifest" in page
+        assert 'data-override="hide"' in page
     finally:
         server.shutdown()
         server.server_close()