OptionalBOOL_OptionalCIRCLE_OptionalCIRCLE_OptionalCIRCLE_OptionalON_OptionalON_Conway extension (real web-ifc has no per-phase progress surface): throttled structured progress events during OpenModel/OpenModelAsync — see core/progress.ts and conway issue #301. Embedders should feature-detect ('ON_PROGRESS simply ignored by older engines').
Conway extension: fired once, right after the STEP header parses — before the full file parse — with everything the header reveals (file name, schema, originating system, preprocessor, byte size), so embedders can print the model line as early as possible (issue #301).