Allow rendering subpages

[MFAshby]

Use case: I want to generate an RSS feed with the full content of my posts embedded in the RSS XML file. Here is my superhtml template:

<rss version="2.0" 
  xmlns:atom="http://www.w3.org/2005/Atom"
  xmlns:content="http://purl.org/rss/1.0/modules/content/">
  <channel>
    <title>Home on mfashby.net</title>
    <link>https://mfashby.net/</link>
    <description>Recent content in Home on mfashby.net</description>
    <language>en-gb</language>
    <lastBuildDate>Sun, 03 Mar 2024 19:31:27 +0000</lastBuildDate>
    <atom:link href="https://mfashby.net/index.xml" rel="self" type="application/rss+xml"/>
    <item :loop="$page.subpages()">
      <title :text="$loop.it.title"></title>
      <link :text="$loop.it.link()"></link>
      <pubDate :text="$loop.it.date.formatHTTP()"></pubDate>
      <guid isPermaLink="true" :text="$loop.it.link()"></guid>
      <description :text="$loop.it.description"></description>
      <content:encoded :html="$loop.it.content()"></content:encoded> 
    </item>
  </channel>
</rss>

However if I try to use this, I get the following error which indicates what I want to do is unsupported:

---------- SCRIPT RESULT TYPE MISMATCH ----------
A script evaluated to an unexpected type.

This attribute expects to evaluate to one
of the following types:
   - string
   - int

[script_eval_not_string_or_int]
(rss.xml) /home/martin/dev/mfashby.net/layouts/rss.xml:16:23:
    <content:encoded :html="$loop.it.content()"></content:encoded>
                     ^^^^^

note: value was generated from this sub-expression:
(rss.xml) /home/martin/dev/mfashby.net/layouts/rss.xml:16:39:
    <content:encoded :html="$loop.it.content()"></content:encoded>
                                     ^^^^^^^
Scripty evaluated to type: err
Error message: 'only the main page can be rendered for now, sorry!'

trace:
    layout `rss.xml`,
    content `posts/index.smd`.