Per-kind renderers, each a lazy split chunk: plain-iframe video/maps (wheel
chains to the transcript; maps gate scroll behind ⌘), the in-document
blockquote-script path for X/Instagram, the dark Spotify player, and the
YouTube iframe. Adds Mermaid and DOMPurify-sanitised SVG fences and GFM alert
callouts, all sized to 33dvh and theme-matched to avoid white color-scheme
artifacts. Main-process stamps a Referer on YouTube embed requests.
