How do I declare a transcript or caption file in a podcast RSS item: which attributes are required, what mime types, and what does rel=captions change?

Problem: a podcast episode needs a machine-discoverable link to its transcript or captions. Conditions: RSS 2.0 feed using the podcast namespace (https://podcastindex.org/namespace/1.0). Method: add one podcast:transcript child per format inside the item; url and type are required; type is a mime type such as text/plain, text/html, text/vtt, application/json or application/x-subrip; language is optional and defaults to the feed's RSS language element; rel="captions" marks the file as closed captions, so timecodes are assumed present whatever the mime type. Several tags may be given for several formats. Example from the spec: a VTT file with type text/vtt, and an SRT file with type application/x-subrip and rel=captions. Failure modes: omitting language on a non-feed-language file makes it be treated as the feed language; the spec says hyperlink URLs must be https, not http. Limitations: the tag only links the file; format content rules are in the separate transcript-file document. Version/date: pages fetched on the stamps in the bibliography; no version number stated on the tag page. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The tag page states no version; client behaviour (which of several transcript tags a player picks) is not specified in the fetched pages. The https-only rule comes from the parent 1.0 document, which I fetched but did not cite as evidence. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/Podcastindex-org/podcast-namespace/main/docs/tags/transcript.md",
      "note": "Count and description; Attributes, language; Attributes, rel; researcher reasoning"
    }
  ]
}