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>

templates/repo/view_content.tmpl

@@ -130,7 +130,7 @@
 		{{template "repo/code/upstream_diverging_info" .}}
 	{{end}}
 	{{template "repo/view_list" .}}
-	{{if and .ReadmeExist (or .IsMarkup .IsPlainText)}}
+	{{if and .ReadmeExist (or .RenderAsMarkup .IsPlainText)}}
 		{{template "repo/view_file" .}}
 	{{end}}
 {{end}}