Open the deck (a default deck if the engine has none yet).
Open the deck (a readable alias of PresentationEngine.open).
Open a deck from encoded bytes. "webdeck" is the native lossless container; "json"
is the human-readable deck.json scene. PPTX/ODP/Keynote import runs in the separate
wd_io_engine module (a later JS bridge), not this editor module.
Optionalformat: "webdeck" | "json"
The presentation surface a WebDecks engine module exposes.