fix(slideshow): retry init at DOMContentLoaded when the children were not parsed yet (#3750)

connectedCallback defers init to a setTimeout(0) so the parser can append
the element's children first. With the bundle loaded from <head>, as
`hyperframes present` emits it, headless Chromium fires that timer while
the parser is still inside <hyperframes-slideshow>: init finds no player
or a half-streamed island, returns, and the deck never binds.

While document.readyState is "loading", init now retries once at
DOMContentLoaded. After parsing there is nothing to wait for, so a missing
or malformed island still fails quietly as before. The subtree check is
split out as locateSlideshowParts so the classification is testable
without the element lifecycle.