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>

routers/web/repo/view_readme.go

@@ -195,16 +195,16 @@ func prepareToRenderReadmeFile(ctx *context.Context, subfolder string, readmeFil
 	}).WithRelativePath(readmeFullPath)
 	renderer := rctx.DetectMarkupRenderer(buf)
 	if renderer != nil {
-		ctx.Data["IsMarkup"] = true
+		ctx.Data["RenderAsMarkup"] = "markup-inplace"
 		ctx.Data["MarkupType"] = rctx.RenderOptions.MarkupType
 		ctx.Data["EscapeStatus"], ctx.Data["FileContent"], err = markupRenderToHTML(ctx, rctx, renderer, rd)
 		if err != nil {
 			log.Error("Render failed for %s in %-v: %v Falling back to rendering source", readmeFile.Name(), ctx.Repo.Repository, err)
-			delete(ctx.Data, "IsMarkup")
+			delete(ctx.Data, "RenderAsMarkup")
 		}
 	}
 
-	if ctx.Data["IsMarkup"] != true {
+	if ctx.Data["RenderAsMarkup"] == nil {
 		ctx.Data["IsPlainText"] = true
 		content, err := io.ReadAll(rd)
 		if err != nil {