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}}

templates/repo/view_file.tmpl

@@ -40,7 +40,9 @@
 				{{if .IsRepresentableAsText}}
 				<a href="?display=source" class="ui mini basic button file-view-toggle-source {{if .IsDisplayingSource}}active{{end}}" data-tooltip-content="{{ctx.Locale.Tr "repo.file_view_source"}}">{{svg "octicon-code" 15}}</a>
 				{{end}}
+				{{if .HasSourceRenderedToggle}}
 				<a href="?display=rendered" class="ui mini basic button file-view-toggle-rendered {{if not .IsDisplayingSource}}active{{end}}" data-tooltip-content="{{ctx.Locale.Tr "repo.file_view_rendered"}}">{{svg "octicon-file" 15}}</a>
+				{{end}}
 			</div>
 			{{if not .ReadmeInList}}
 				<div class="ui buttons tw-mr-1">
@@ -90,15 +92,15 @@
 	</h4>
 
 	<div class="ui bottom attached table unstackable segment">
-		{{if not .IsMarkup}}
+		{{if not .RenderAsMarkup}}
 			{{template "repo/unicode_escape_prompt" dict "EscapeStatus" .EscapeStatus}}
 		{{end}}
-		<div class="file-view {{if .IsMarkup}}markup {{.MarkupType}}{{else if .IsPlainText}}plain-text{{else if .IsDisplayingSource}}code-view{{end}}">
+		<div class="file-view {{if eq .RenderAsMarkup "markup-inplace"}}markup {{.MarkupType}}{{else if .IsPlainText}}plain-text{{else if .IsDisplayingSource}}code-view{{end}}">
 			{{if .IsFileTooLarge}}
 				{{template "shared/filetoolarge" dict "RawFileLink" .RawFileLink}}
 			{{else if not .FileSize}}
 				{{template "shared/fileisempty"}}
-			{{else if .IsMarkup}}
+			{{else if .RenderAsMarkup}}
 				{{.FileContent}}
 			{{else if .IsPlainText}}
 				<pre>{{if .FileContent}}{{.FileContent}}{{end}}</pre>