Frontend iframe renderer framework: 3D models, OpenAPI (#37233)

Introduces a frontend external-render framework that runs renderer
plugins inside an `iframe` (loaded via `srcdoc` to keep the CSP
`sandbox` directive working without origin-related console noise), and
migrates the 3D viewer and OpenAPI/Swagger renderers onto it. PDF and
asciicast paths are refactored to share the same `data-render-name`
mechanism.

Adds e2e coverage for 3D, PDF, asciicast and OpenAPI render paths, plus
a regression for the `RefTypeNameSubURL` double-escape on non-ASCII
branch names.

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/external/external.go

@@ -21,7 +21,33 @@ import (
 
 // RegisterRenderers registers all supported third part renderers according settings
 func RegisterRenderers() {
-	markup.RegisterRenderer(&openAPIRenderer{})
+	markup.RegisterRenderer(&frontendRenderer{
+		name: "openapi-swagger",
+		patterns: []string{
+			"openapi.yaml",
+			"openapi.yml",
+			"openapi.json",
+			"swagger.yaml",
+			"swagger.yml",
+			"swagger.json",
+		},
+	})
+
+	markup.RegisterRenderer(&frontendRenderer{
+		name: "viewer-3d",
+		patterns: []string{
+			// It needs more logic to make it overall right (render a text 3D model automatically):
+			// we need to distinguish the ambiguous filename extensions.
+			// For example: "*.amf, *.obj, *.off, *.step" might be or not be a 3D model file.
+			// So when it is a text file, we can't assume that "we only render it by 3D plugin",
+			// otherwise the end users would be impossible to view its real content when the file is not a 3D model.
+			"*.3dm", "*.3ds", "*.3mf", "*.amf", "*.bim", "*.brep",
+			"*.dae", "*.fbx", "*.fcstd", "*.glb", "*.gltf",
+			"*.ifc", "*.igs", "*.iges", "*.stp", "*.step",
+			"*.stl", "*.obj", "*.off", "*.ply", "*.wrl",
+		},
+	})
+
 	for _, renderer := range setting.ExternalMarkupRenderers {
 		markup.RegisterRenderer(&Renderer{renderer})
 	}